NEW YORK (CNN/Money) - The number of millionaires in America reached record highs in 2004, hitting 7.5 million, according to a new survey.

That represented a gain of 21 percent, the largest jump in the number of U.S. millionaires since 1998, according to the survey by the Spectrem Group, a Chicago-based research firm.

Spectrem counted Americans with net assets of $1 million or more, excluding primary residences but including second homes and other real estate holdings. There were 6 million millionaires in 2001, when the bursting of the tech-stock bubble pruned more than a million Americans from this status.

The firm surveyed data from 450 households. It found the ranks of the very wealthiest Americans, those with investable assets of more than $5 million, climbed even faster, up 38 percent in 2004.

Much of the increased wealth can be traced to stock market gains, according to the report. About 37 percent of the investable assets of Americans with net worth of $500,000 or more are in stocks and mutual funds and another 14 percent are in IRAs. About 9 percent is in cash.

The survey found that more than 35 percent of the affluent are retired and 36 percent are business owners. The overwhelming majority – 86 percent – are married, and the mean age is just under 56.

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