A London-based market-research company, TNS Global, says King County now has 68,000 millionaires.

That places it number 10 in the country in millionaire residents. TNS defines millionaires as people with a net worth of $1 million or more, excluding their primary residences.

CNN Money has a photo-gallery of the top 10 counties.

The TNS report says U.S. millionaires last year numbered 9.3 million, a 5 percent increase over the year before.

Los Angeles County had more millionaires than any other, 268,136, and California had four of the top 10 counties.

According to TNS, King County, which according to Census estimates accounts for about 29 percent of the state's population, had 33 percent of Washington's millionaires. This suggests that the state has about 204,000 millionaires, or 3.2 percent of the 6.3 million residents.

It was King County's first appearance on the top 10 list. It displaced Middlesex County, Mass.

CNN Money also has aerial photos of the mansions of eight of America's richest people, including Bill Gates and Paul Allen, based on information from Zillow.com.