U.S. retirement plans held $13.4 trillion in assets March 31, down about 22% from $17.1 trillion at the end of the first quarter of 2008, according to the Investment Company Institute.

Retirement savings accounted for 33% of all U.S. household financial assets at the end of the first quarter.

The ICI, Washington, has based those estimates on data from its own database, the Federal Reserve and other sources.

Declines in financial markets led to asset drops at all types of retirement plans, with individual retirement accounts, defined contribution plans, private defined benefit plans and state and local government pension plans all suffering similar percentage declines over this period, the ICI says.

Americans held $3.4 trillion in employer-based defined contribution retirement plans at the end of the first quarter. Mutual funds accounted for $1.5 trillion, or 45%, of DC plan assets.

Individual retirement accounts held $3.4 trillion at the end of the first quarter, and 44% of IRA assets, or $1.5 trillion, were in mutual funds.