Average 401(k) account balances held with Fidelity Investments went up 28% last year, allowing many workers to recoup much of their losses from 2008, according to the company.

Fidelity, the nation's largest provider of workplace retirement plans, also noted in its year-end report that average deferral rates remained flat for the year at about 8.2%.

"The good news is that many workers, in spite of the economy, chose to save in their 401(k)s throughout 2009, and as the markets recovered, so did many Americans' account balances," said Jim MacDonald, president of Workplace Investing at Fidelity Investments.

During the 10-year period spanning 1999 to 2009, investors with Fidelity retirement accounts for the entire time saw their account balances grow 150%, to an average of $163,900. This group of participants had a median age of 51 years and a deferral rate of 10.4%.

The report also found that risk-taking did not pay off during the 10-year period. About 65% of investors in the 10-year group too higher risks with their accounts than Fidelity's age-based Freedom Fund. Of the risk takers, 69% showed a lower return than the age-based fund.