Home prices in 20 metropolitan cities in the United States rose in May, recording the biggest year-over-year increase since August 2006.

According to the Standard and Poor’s/Case-Shiller index of property values, prices rose 4.6 percent from a year ago in the 20-city composite in May.

Prices are expected to drop slightly in the future, because demand for housing will go down now that a tax credit for first-time home buyers has expired, Bloomberg reported.

David Blitzer, chairman of Standard & Poor’s index committee, said in a statement that housing isn’t in “any form of sustained recovery” although prices were better than in April.

“Since reaching its recent trough in April 2009, the housing market has really only stabilized at this lower level,” he said. “The two composites have improved between 5 and 6 percent since then, but this is no better than the improvement they had registered as of October 2009. The last seven months have basically been flat.”

Another report released today by the U.S. Census Bureau showed that more people are renting apartments; the vacancy rate has dropped from 8.2 percent to 6.6 percent since December. Read more.