Counter to a national trend, the ranks of millionaire households surprisingly declined in L.A. County last year, according to figures released Monday from a global market research firm.

There were an estimated 261,081 millionaire households in L.A. County as of June 2007, according to annual data from London-based TNS Financial Services affluent market research program. While that keeps L.A. on top in terms of the overall number of millionaire households in the U.S, it was down 2.6 percent from June 2006 and somewhat from June 2005.

Nationwide, the number of millionaire households increased 5.9 percent to 9.9 million in 2007. L.A. County’s share of that total was 3 percent, followed by Cook County, Ill. (home to Chicago) and Maricopa County, Ariz. (Phoenix). Other California counties among the top 10 included No. 4 Orange County (115,396), No. 6 San Diego County (100,727) and No. 8 Santa Clara County (72,932).

A spokeswoman for TNS Financial Services said the researchers there had no comment or explanation on why L.A.’s millionaire households fell.

However, Jack Kyser, chief economist with the Los Angeles County Economic Development Corp., questioned whether the data was accurate.

“In June 2007, our local economy was basically running on all thrusters, with the only sign of trouble being from the subprime industry meltdown and that was largely concentrated in Orange County,” Kyser said.

The study is based on a national sample of more than 3,000 households with a net worth of at least $500,000, excluding primary residence.