The survey by the Spectrem Group, a Chicago-based consulting firm specializing in the affluent and retirement markets, also found that the number of households with a net worth of $5 million or more — what Spectrem calls "Ultra High Net Worth households" — rose to 930,000 in 2005.
The net worth figures — which reflect a household's assets minus its liabilities — exclude primary residences.
"It's been a great couple of years for America's millionaires," Catherine S. McBreen, Spectrem's managing director, said in a statement accompanying the report. "Clearly, the stock market, which posted solid improvement in 2005, was one reason for the advance."
She added that "for the wealthiest Americans, it appears the increased use of international markets and alternative investments were key drivers of their improvement."
The report, titled "Affluent Market Insights 2006," was based on data gathered through mail and online surveys last fall from 1,014 people. The margin of error was plus or minus 3.1 percentage points.