NEW YORK (CNN/Money) - The number of millionaire households in the United States by mid-year 2003 grew to the highest level in 20 years, a study said Tuesday.

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.

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Since NFO's study is intended for its clients who wish to attract wealth-holders' assets, the "investable assets" measured by the study do not include those assets that an owner is unlikely to move, such as a vacation home, a 401(k) or 403(b), stock options, investment real estate, annuities, UGMA accounts or closely held business partnerships.

If those are included and household net worth is measured -- equal to assets minus liabilities -- then the number of millionaire households jumps to 7.9 million, up from 7.4 million in 2002.

But that's still far below the all-time high of 9.8 million households reached in 2001.

Also on the upswing this year were the number of affluent households with a net worth of $500,000 or more, excluding primary residences. They jumped to 10.5 million households from 9.1 million in 2002, NFO found.

Luhr attributed the rise in high-net-worth households to several factors: diversified portfolios, professional guidance and a healthy amount of real estate exposure.

The percentage of total assets that affluent households allocated to real estate, excluding primary residences, doubled in the past year, NFO found.

"Low interest rates and rising property values have had a profound impact on these households," Luhr said in a statement.

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NFO's Affluent Market Research Program Confidence Index, measuring affluent Americans' outlook on the economy and their own financial situations, showed a slight increase from 2002, but respondents' optimism was not evenly distributed.

When asked by NFO how confident they felt about the stock market, half of the millionaire households felt the broad market and the Nasdaq would continue to improve over the next six months, up from 36 percent who felt the same way a year earlier.

They were less confident, however, about the prospects in their own financial lives. Only 35 percent felt their household income and finances would improve, versus 55 percent a year ago.

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The NFO study comes after a government report last week found poverty is on the rise for the second straight year and median incomes in the United States fell for the third year in a row.

The percentage of Americans living in poverty rose to 12.1 percent from 11.7 percent in 2001, the Census Bureau reported, the highest rate since 12.9 percent in 1998.

And on Tuesday, the bureau said the number of Americans without health insurance grew by 2.4 million people in 2002, bringing the total number of uninsured to nearly 44 million.