FOR YEARS, investors have grumbled that there aren t enough investment choices in their 401(k) plans and grew even more disgruntled after losing a good chunk of their retirement funds in the crash. So you d think getting access to one of the hottest investment products of the past few years would be good news. Too bad it s more a case of being careful what you wish for.

ETF firms have long been eyeing the $2.3 trillion 401(k) jackpot. But until recently, companies that offered 401(k) retirement plans weren t inclined to include exchange-traded funds. ETFs are baskets of stocks or other securities that trade as a single unit, a structure that can quickly become a record-keeping nightmare for firms that run the plans. That may be about to change, though: Those so-called plan administrators have recently found ways to overcome the record-keeping and trading issues that have hampered ETFs in the past. Already, Barclays, one of the biggest players in the game, has launched a new program to help financial advisers get its iShares ETFs into 401(k) accounts.

The tactic? Aim for 401(k) plans that want more cost-efficient options. Meanwhile, State Street Global Advisors, seller of the famous spider ETFs, is trying to get its foot in the corporate suite by offering retirement-oriented target date ETFs. Target-date funds have already achieved special status in many plans, despite their poor performance in the crash, and State Street aims to play a part in the future, says Anthony Rochte, senior managing director at State Street Global Advisors.

But the introduction of ETFs into 401(k) plans could be a mixed blessing. Typically, ETFs are cheaper than comparable mutual funds, but employees of large companies aren t likely to see lower fees big firms already can get rock-bottom fees on most funds, says Pamela Hess, director of retirement research at Hewitt Associates. Plus, the tax advantages of ETFs are minimized when held in a tax-deferred account like a 401(k). ETFs could still have broad appeal for small and midsize firms looking for lower-cost investments to include in their 401(k) plans. Nevertheless, says Rick Meigs, president of 401khelpcenter.com, there s no groundswell of demand for ETFs by 401(k) plan sponsors.