For decades, financial advisors have preached the same lesson: buy great companies, stay invested, and let compounding do the work.

Millions of Americans followed that advice.

Now, many of them have a problem they never expected.

After years of historic gains in technology stocks and broad-market index funds, investors are finding themselves with portfolios dominated by just a handful of positions. Selling those winners to diversify may be the financially prudent move, but it can also trigger capital gains taxes large enough to make investors think twice.

That growing dilemma has fueled interest in a little-known strategy that allows certain investors to diversify concentrated stock positions while postponing a potentially enormous tax bill.

For affluent investors, retirees, and families planning generational wealth transfers, it has become one of the fastest-growing trends in ETF investing.

Bull Market Winners Can Become Portfolio Risks

Some of today’s largest fortunes were built simply by refusing to sell.

An investor who purchased roughly $1,500 of Apple (NASDAQ: AAPL) stock in early 2001 would now own shares worth approximately $2.2 million, including dividends.

Someone who inherited $100,000 of Nvidia (NASDAQ: NVDA) stock in early 2019 would now hold a position valued at roughly $5.3 million.

While those returns are extraordinary, they also illustrate an uncomfortable reality.

A portfolio that becomes heavily concentrated in one or two companies exposes investors to risks that have nothing to do with taxes.

Corporate leadership changes.

Competitive pressures emerge.

Technology shifts.

Entire industries evolve.

Even dominant businesses eventually face periods of underperformance.

The problem is that reducing those concentrated positions can come with a tax bill that discourages investors from making what might otherwise be a sensible investment decision.

When Taxes Start Driving Investment Decisions

Financial planners often refer to this phenomenon as tax lock.

Instead of asking, “What allocation gives me the best long-term risk-adjusted returns?” investors begin asking, “How can I avoid paying taxes?”

The distinction matters.

Someone living in a high-tax state could face combined federal and state capital gains taxes that consume a significant portion of decades of accumulated gains.

That concern has become increasingly common among investors who purchased index funds years ago as well.

Because the largest technology companies now represent such a large percentage of major indexes, many long-term investors unknowingly have substantial exposure to the same handful of mega-cap stocks across multiple funds.

The result is a growing number of portfolios where taxes—not investment strategy—become the primary obstacle to diversification.

A Little-Known ETF Strategy Is Gaining Momentum

One solution attracting increasing attention is known as a 351 exchange.

Unlike a traditional sale, investors contribute appreciated securities into the launch of a newly created exchange-traded fund.

In return, they receive shares of that diversified ETF rather than cash.

Because the transaction generally qualifies for tax deferral under Section 351 of the Internal Revenue Code, investors can often postpone realizing capital gains while immediately gaining broader diversification.

Instead of owning a single stock directly, they own shares in a diversified investment vehicle that contains many holdings.

The original tax basis transfers to the ETF shares, meaning taxes generally are deferred until those ETF shares are eventually sold.

Why Wealth Managers Are Paying Attention

The strategy has quietly become one of the fastest-growing niches in the ETF industry.

According to industry data, more than 80 ETFs have launched using 351 exchanges since 2021, collectively attracting over $18 billion in initial assets.

Additional launches are expected as demand continues to grow.

The appeal extends across the investment industry.

ETF sponsors gain significant assets under management from day one.

Financial advisors gain another tool for helping clients reduce concentration risk.

Investors gain access to diversification without immediately triggering a potentially seven-figure tax bill.

As portfolios continue growing, many advisors expect these transactions to become increasingly common among high-net-worth households.

Not Everyone Can Use This Strategy

Despite the growing popularity, 351 exchanges remain subject to strict rules.

Among the most important limitations:

A single company’s stock generally cannot represent more than 25% of the contributed portfolio.
The five largest holdings generally cannot exceed 50% of contributed assets.
Investors can participate only during the creation of a new ETF, not after it has already begun trading.

Ironically, investors with the largest winning positions may have the hardest time qualifying.

Someone whose wealth is overwhelmingly concentrated in a single stock may exceed federal diversification limits before the transaction even begins.

For many investors, that means planning years in advance rather than waiting until a position becomes extraordinarily large.

Diversification Isn’t the Only Option

A 351 exchange is only one tool available for managing concentrated stock positions.

Depending on an investor’s goals, other strategies may include:

Gradually selling shares over multiple tax years.
Donating appreciated securities to qualified charities.
Using donor-advised funds to combine charitable giving with tax planning.
Offsetting gains with capital losses from other investments.
Holding appreciated assets as part of an estate, where current law generally provides heirs with a step-up in cost basis.

Each approach involves different tradeoffs involving taxes, liquidity, investment flexibility, and estate planning.

That’s why advisors generally recommend evaluating concentrated positions within the context of an investor’s broader financial plan rather than focusing solely on minimizing taxes.

The Bigger Risk May Be Doing Nothing

Avoiding taxes feels rewarding.

But avoiding portfolio risk can be even more valuable over the long run.

History offers countless examples of companies that once appeared untouchable before years of underperformance changed the picture. While today’s market leaders may continue thriving, relying too heavily on a single company or sector introduces risks that many investors underestimate during long bull markets.

The decision ultimately becomes a balancing act.

Paying taxes unnecessarily can reduce wealth.

Refusing to diversify because of taxes can increase investment risk.

The right answer depends on an investor’s overall financial picture, risk tolerance, income needs, and estate planning objectives.

Why This Trend Is Likely to Grow

The extraordinary performance of U.S. equities over the past two decades has created a generation of investors with highly appreciated portfolios.

As Baby Boomers retire and wealth increasingly transfers to younger generations, tax-efficient diversification strategies are likely to receive even greater attention.

At the same time, the continued expansion of the ETF industry is creating more opportunities for specialized strategies like 351 exchanges.

Whether this particular structure becomes mainstream remains to be seen.

What is clear is that taxes are becoming a much larger part of portfolio management conversations than they were a decade ago.

For investors fortunate enough to have accumulated substantial unrealized gains, preserving wealth is no longer just about choosing the next winning investment. It’s increasingly about finding the smartest way to manage the winners they already own.

Investor Takeaway

A 351 exchange can be an effective strategy for investors with highly appreciated stock positions who want to diversify without immediately realizing capital gains. However, the rules are complex, eligibility is limited, and participation is generally restricted to the launch of new ETFs. Investors considering this approach should work closely with qualified tax and financial advisors to determine whether it fits their overall investment, tax, and estate planning goals.