NEW YORK (CNNMoney.com) -- Another year, another record for the rich. The ranks of households with a net worth of at least $1 million grew 5 percent to an all-time high of 9.3 million, according to a report released Monday.
The report, by research firm TNS, covers the year through mid-2006; the value of primary residences was not included.
The number of millionaire households has increased in each of the past four years, with the biggest growth spurt occurring in 2004, when the number of high-net-worth households jumped 33 percent.
As was the case last year, four of the top 10 counties with the greatest number of millionaire households are in California. Combined, those four California counties boast 561,255 millionaire households.
Other states with counties that boast the highest number of millionaires across the country are Illinois, Arizona, Texas, New York, Florida and Washington. King County, Wash., bumped off Middlesex County, Mass., to take 10th place this year.
TNS found that the average net worth of millionaire households was $2.5 million, of which more than half was in investable assets. These households' debt levels had risen from an average of $175,238 in 2005 to $208,558 in 2006.
The median age of the heads of these households was 59, and 15 percent of them owned or co-owned their own business or practice. Almost half (46 percent) were already retired, while another 16 percent were semi-retired.
The ranks of the wealthy with even higher net worths than $1 million are also expanding. A recent survey by the Spectrem group found that the number of U.S. households with a net worth of $5 million or more, excluding primary residences, exceeded 1 million for the first time. 
