The trade group said sales of single-family homes, townhomes, condominiums and co-ops fell to a seasonally adjusted annual rate of 4.55 million in May, down from a revised figure for April of 4.62 million.
Sales for the month were 9.6 percent higher than the 4.15 million level of May 2011.
"Even with the monthly decline, home sales have moved markedly higher with 11 consecutive months of gains over the same month a year earlier," said NAR chief economist Lawrence Yun.
Data also shows that the median sales price for existing homes, which jumped 10.1 percent in April, climbed another 7.9 percent in May to $182,600.
The median price has climbed three consecutive months, the first time that has happened since March through May of 2006, NAR said.
"The recovery is occurring despite excessively tight credit conditions and higher down payment requirements, which are negating the impact of record high affordability conditions," Yun said.
NAR said sales of existing homes fell 4.8 percent in the Northeast from April to May, but rose 7.3 percent compared with May 2011.
Existing home sales in the Midwest rose 1 percent from the previous month and 19.5 percent from the same month of 2011.
In the South, sales slid 0.6 percent month to month and rose 9.2 percent above May 2011. In the West sales were down 3.4 percent from April, but up 3.6 percent from a year earlier.
At the end of the month, there were 2.49 million existing homes on the market, a 6.5-month supply at the current rate of sales, NAR said.
The inventory of homes for sale is a steep 20.4 percent below May 2011, the trade group said.
At the end of March, existing homes on the market represented a 6.2-month supply based on the current rate of sales.
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