WASHINGTON — Individual retirement accounts held $4.8 trillion in assets as of September 2007 boosting Americans' overall retirement assets to $17.8 trillion, according to the Investment Company Institute.

The association of mutual funds and other investment companies recently released its latest quarterly statistical series that showed retirement savings account for almost 40% of all household financial assets in the United States. Mutual funds managed $2.4 trillion in assets in 401(k), 403(b) and other defined contribution plans as of third quarter 2007, up from $2.3 trillion at the end of the second quarter, according to ICI. Mutual funds also managed 53% of defined contribution assets.

Lifecycle funds, which typically rebalance their portfolios to include more fixed-income assets as a specified target date approaches, continued to grow rapidly, the data showed. They managed $168 billion at the end of the third quarter of 2007, compared with $153 billion three months earlier. Nearly 90% of assets in lifecycle funds are in retirement accounts.