The report showed the median home price in April was down 8 percent from a year ago, at $202,300.
It was the second-largest price decline on record, following the biggest drop in February.
"The big surprise was the inventory of unsold homes rising to a record level,' said Rudy Narvas, a senior analyst at 4Cast Ltd.
in New York.
"This would suggest to us that further price declines are going to be necessary for the inventory to clear." A report Thursday showed home price declines accelerated in the first quarter.
The federal Office of Housing Enterprise Oversight said its price index fell 1.7 percent in the first quarter, the steepest drop in the index's 17-year history.
Other price measures have shown even steeper drops.
The Standard & Poor's/Case Shiller home price index of 20 metropolitan areas showed a drop of 12.7 percent in the 12 months through February, with prices down 15.8 percent from their June 2006 peak.
The March index will be released Tuesday.
"With prices collapsing, the incentive not to buy a home is increasing by the week, and with inventory showing no sign of improvement prices will keep falling," said Ian Shepherdson, chief U.S. economist at High Frequency Economics in Valhalla, New York.
Moody's Zandi said about one-fourth of the sales likely were due to foreclosure, which he said was another negative sign.
NAR Chief Economist Lawrence Yun said that foreclosed homes, which sell at substantially lower prices, were increasingly showing up in the existing home sales data.
"Several markets are seeing a significant rise in home sales," Yun said.
"These markets are also the markets that have witnessed a substantial decline in prices." The trade association said last month's existing home sales pace was 17.5 percent below the rate of April 2007, with single-family home sales off 16.1 percent and sales of multiple family units down 27.9 percent.