(Stock prices and Lennar downgrade added.)NEW YORK ( TheStreet) -- Home prices posted their first monthly increase in three years, adding to the growing list of positive housing data. The Standard & Poor's/Case-Shiller home price index of 20 major cities rose 0.5% in May from April, the first monthly gain since July 2006. While prices were still 17.1% below last year, 13 metro areas showed signs of improvement, compared with eight in April. The biggest winner was Cleveland, where prices rose 4.1%. New York and Tampa, Fla., remained unchanged, while Las Vegas was the biggest loser, seeing prices fall 2.6%. Home prices are just another hint that the worst of the housing-market mess could be over. On Monday, the Commerce Department said new home sales grew 11% in June, and last week the National Association of Realtors reported existing home sales jumped 3.6% in June. But shares of home builders were in the red Tuesday morning, after one of their brethren, Lennar ( LEN), was downgraded by Citi. Analyst Josh Levin lowered his rating on Lennar to hold from buy, saying its shares have grown more than its competitors and the overall market. While Levin doesn't foresee a slowdown in the near future, he says the sector still remains challenged. Shares of Lennar fell 3% to $11.54 in morning trading, Toll Brothers ( TOL) dropped 0.5% to $19.21 and NVR ( NVR), shed 1% to $614.25. --Reported by Jeanine Poggi in New York.