Total U.S. retirement assets rose to a record $16.6 trillion in the first quarter of this year, up from $16.4 trillion at year-end 2006, according to a new statistical series launched today by the Investment Company Institute (ICI), the national association of U.S. mutual funds and other investment companies.

According to the ICI, retirement savings account for almost 40% of all household financial assets in the U.S. IRAs held more than $4.3 trillion in the first quarter of this year, up from $4.2 trillion at year-end 2006, while 401(k) plans showed $2.75 trillion in assets, compared to $2.7 trillion at the end of 2006. The ICI counted $4.16 trillion in all employer-based defined contribution plans at the end of the first quarter, up from $4.08 trillion for the whole of 2006.

ICI, which for many years has published the earliest comprehensive releases on year-end retirement assets, will now update total and detailed figures on retirement savings on a quarterly basis.

The findings for the first quarter of 2007 also determined that lifecycle funds continued to grow in popularity, managing $133 billion as compared to $114 billion at year-end 2006. Lifestyle funds, which mix equity and fixed-income investments to maintain predetermined risk level, managed $204 billion at the end of March 2007, up from $189 billion at the end of December 2006.