Although assets in 401(k) and IRA retirement plans have risen significantly in recent years, growth in the number of these plans as well as in the number of participants in them has slowed down considerably in recent years, according to a recent study conducted by the Washington, D.C.-based Employee Benefits Research Institute (EBRI).

IRAs and defined contribution plans held $7.5 trillion in assets at the end of 2006, according to Craig Copeland, EBRI senior research fellow and author of the study, but even so, “the data also show that a majority of Americans still do not have a retirement plan.”

According to the study, IRAs have become the single largest vehicle for retirement assets in the U.S., and their growth continues to be a result of rollovers from other tax-qualified retirement plans, rather than from new contributions. Within the IRA universe, Roth IRAs are growing the most, but traditional IRAs still hold the bulk of IRA assets.

The new challenge for retirees who own IRAs and 401(k)s and have accumulated significant savings in them will be to maintain and draw down the account balances in a way that pays for an acceptable standard of living but still lasts throughout the length of their retirement, the EBRI study states.