Inventory of existing homes for sale in 18 major U.S. markets surged by 6.52% in March over February, according to data complied by Zip Realty, the Emeryville, Calif.-based national real estate brokerage. A total of 693,706 homes, defined as single-family houses, condominiums and town houses, were on the market in March, up from 651,259 the month before and from 429,150 in the same month last year, the latter marking a rise of 35%.

Los Angeles led all markets with a sequential rise of 12.8% to 93,830. Compared to a year earlier, listings were up 54%. Second was the Bay Area, with listings up 12.2% to 24,311, a 39.7% increase year over year. Third with a 9.4% increase was Washington D.C., with 37,973 homes for sale, an year-over-year increase of 21.4%. Fourth was Orange County, Calif., with a 9.3% sequential increase to 13,995, up 36.4% from last year, and fifth was Las Vegas, with an 8.1% increase to 24,523, up 47.2% from last year.

Among other markets, Miami was up only 1.8% sequentially to 72,672 but up 61.1% compared to March 2006. Other cities with large year-to-year increases included Seattle (+54.3%); Phoenix (+49.6%); Chicago (+38.3%) and Baltimore (+38.2%). Sequentially, those markets posted increases between 4% and 8%.

Not one of the 18 markets posted a decrease in inventory.

A spokesman for Zip said that much of the increase can likely be attributed to people putting their homes on the market earlier than usual as they anticipate longer times on the market in the current housing-sales environment.

Zip compiles the data based on homes listed on multiple listing services in the 18 markets.

Learn more about markets featured in this article: Los Angeles, CA.