The ranks of millionaire households jumped 14 percent to 3.8 million, up from 3.3 million in 2002 and 3.7 million in 2001, according to NFO WorldGroup, a market research and consulting firm, which conducts an annual survey of affluent households.
NFO defines a "millionaire" household as one having $1 million or more in investable assets, which doesn't include primary residences or 401(k)s, among other things.
The 3.8 million was the highest in the 20 years that NFO has been conducting its surveys.
The increase in households with $1 million or more in investable assets is due largely to increased stability in the stock market and to well-diversified portfolios, said Jeanette Luhr, NFO Financial Services Director and program manager for the study.
While the number of millionaire households is at a 20-year high, it still accounts for just 3.4 percent of the 111 million households in the United States.