Stock market performance accounted for about 80 percent of the average $5,500 increase in the first quarter of 2012 compared with the prior quarter, while the remaining 20 percent was from employer and worker contributions, Fidelity said. The S&P 500 rose about 12 percent in the first three months of this year.

The growth in the accounts comes as Americans continue to say they're worried about outliving their savings and Congress weighs limiting contributions or reducing the tax advantages of the plans.

Workers' confidence in their ability to retire remains historically low, with about 14 percent saying they were very certain they'd have enough to live on comfortably, according to the Employee Benefit Research Institute, a Washington-based nonprofit that studies employee benefits, in a March survey. That compares with a high of 27 percent in 2007.

Capping Contributions

The U.S. House Ways and Means Committee held a hearing in April on retirement savings as Congress seeks to overhaul the nation's tax code and reduce the deficit. Proposals to alter the benefits have included reducing the amount that can be contributed or replacing deductions for savings with credits. Tax incentives for retirement accounts cost the government more than $130 billion in revenue this year, according to the Joint Committee on Taxation.

A 401(k) plan generally lets employees defer a portion of their wages to the account on a pretax basis. Contributions are limited to $17,000 for 2012 and those age 50 or older may set aside an additional $5,500, according to the Internal Revenue Service.

Legislators and regulators from the Departments of Labor to Treasury have been looking at Americans' retirement security because life expectancies are increasing and savings have shifted from traditional pension plans, where employers generally provided retired employees with lifetime payments, to 401(k) accounts that individuals largely are responsible for funding.

About 60 percent of U.S. workers said they have less than $25,000 in savings and investments, according to the EBRI survey.

Fidelity, the largest provider of 401(k)s, has almost 12 million participants in about 20,000 employer-sponsored defined contribution plans. The mutual-fund manager changed its average balance calculation last year to include 401(k) accounts that are sold through advisers as well as directly through the company, McHugh said. Using the prior methodology, average account balances were $74,900 at the end of the first quarter last year, Fidelity data show.

About 10 percent of workers increased their savings rate during the first quarter of this year, compared with 4 percent who decreased it, according to the report. The corresponding percentages in the three months ended March 31, 2009, were 5.7 percent who increased their savings rate and 6.4 percent who decreased it, McHugh said.

“Even with the market and its unpredictability we're not necessarily seeing people shy away from contributing,” said McHugh, who is based in Covington, Kentucky. Participants saved 8 percent on average of their salaries, which was unchanged from last year, she said.

The majority of plans administered by Fidelity offer workers the ability to automatically increase their savings rate each year, usually by 1 percent, said McHugh.

Americans held $3.1 trillion in 401(k)s as of Dec. 31, according to the Washington-based Investment Company Institute, a trade group for the mutual-fund industry.

The Fidelity data on average balances looks at single accounts and doesn't include additional savings workers may have in individual retirement accounts or multiple 401(k)s, said McHugh. The median account balance at the end of the first quarter was $23,000, Fidelity said.

In 2011 the median balance of accounts administered by Vanguard Group Inc. was $25,550 and the average was $78,276, according to Linda Wolohan, a spokeswoman for the mutual-fund manager based in Valley Forge, Pennsylvania.

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