U.S. retirement assets rose 3% from year-end 2010 to $18.1 trillion as of March 31, the Investment Company Institute said.

At the end of the first quarter, retirement savings accounted for 37% of all household financial assets.

Assets in individual retirement accounts totaled $4.9 trillion at the end of the first quarter, up 3.2% from year-end 2010. Defined contribution plan asses increased 3.6% in the first quarter to $4.7 trillion. Government pension plans held $4.5 trillion in assets at the end of the quarter, up 2.4% from the end of 2010, and private sector defined benefit plans held $2.3 trillion in assets at the end of the first quarter, with annuity reserves held outside retirement accounts totaling another $1.6 trillion.

The total retirement assets of $18.1 trillion are just below their all-time high reached at the end of the third quarter of 2007, the ICI said.