A study by the Center for Responsive Politics, headquartered in Washington, reviewed federal financial disclosures earlier this year.
Though some members of Congress lost money, nearly half of them, 261, are millionaires, the study found, a slight increase from the previous year. Only about 1 percent of Americans in general are millionaires.
The study said 55 congressional millionaires had an average calculated wealth of $10 million or more, while eight weighed in with more than $100 million.
The median wealth of a U.S. House member in 2009 was $765,010, up from $645,503 in 2008, the study said.
Senators were richer. The median wealth of a U.S. senator was nearly $2.38 million, up from $2.27 million in 2008.
The study said for all members of Congress regardless of chamber, median wealth in 2009 was at $911,510, up from $785,515 in 2008.
Averaging lawmakers' minimum and maximum potential wealth for 2009, Rep. Darrell Issa, R-Calif., was at the top of the list with holdings exceeding $303.5 million. Next was Rep. Jane Harman, D-Calif., with $293.4 million. Sen. John Kerry, D-Mass., was third with $238.8 million.