U.S. home prices rose month-to-month for the second time in eight months in May, the closely watched S&P/Case Shiller Home Price Index report said Tuesday. The 10-city Home Price Index rose 1.1 percent, while the 20-city index rose 1 percent April to May. From the same month of 2010, however, home prices remain down in most cities. Sixteen cities had month-to-month increases, while Washington D.C. was the only city with a 12-month increase with prices up 1.3 percent on the year, the report said. Month-to-month, prices in Detroit, Las Vegas and Tampa, Fla., were down, while prices in Phoenix were unchanged. "This is a seasonal period of strong demand for houses, so monthly price increases are to be expected," said index chairman David Blitzer in a statement. With 19 of 20 cities showing a 12-month decline, however, "The concern is that much of the monthly gains are only seasonal," he said.