The number of U.S. millionaire households has risen to a record high of 9.3 million as of mid-2006, up 5 percent from 2005, according to TNS Global’s annual Affluent Market Research Program.

The millionaires’ mean net worth, not including their primary residence, is $2,167,167 with investable assets of $1,442,841. Their median age is 58 and 45 percent are retired.

Forty-six percent of millionaire households own investment real estate such as a second home, third home, rental properties and undeveloped land. Thirty-four percent have a first mortgage on these residences and 25 percent have second mortgages on these additional residences.

The TNS study identified 10 counties with the highest number of millionaire residents.

• Los Angeles County with 268,136
• Cook County, Ill., 171,118
• Orange County, Calif., 116,157
• Maricopa County, Ariz., 113,414
• San Diego County, Calif., 102,138
• Harris County, Texas, 99,504
• Nassau County, N.Y., 79,704
• Santa Clara County, Calif., 74,824
• Palm Beach County, Fla., 71,221
• King County, Ore., 68,390