The average wealth of the 8 million households in the top sector jumped 28 percent to $3.2 million from $2.5 million from 2009 to 2011, a Pew Research Center report said.
But the 111 million households with an average net worth of $862,700 or less -- the bottom 93 percent -- lost 4 percent of their wealth, dropping to $134,000 from $140,000, said the report, which analyzed U.S. Census Bureau data.
Wealth is the value of what a household owns minus what it owes.
The Census Bureau defined the bottom 93 percent in 2009 as households with wealth valued at $889,275 or less and in 2011 as households with wealth valued at $836.033 or less. United Press International averaged the two values to get the average $862,700.
The economic-equality changes mean the top 7 percent were an average 24 times richer than everyone else in 2011, up from 18 times richer in 2009.
Overall, the most affluent 7 percent owned 63 percent of the nation's household wealth, up from 56 percent in 2009.
The gap between the rich and poor, which has grown significantly since the 1970s, helped fuel the Occupy Wall Street movement that began in September 2011, when supporters started setting up encampments in New York's financial district, Washington and elsewhere to protest the financial chasm between the wealthiest 1 percent of Americans and the rest.
The wealth disparity Pew studied was driven by rallies in stock and bond markets during the two years -- the latest for which Census wealth data are available -- compared with a flat housing market, the think tank said.
Wealthy households generally invest in stocks and other securities while less-affluent households typically define their wealth by the value of their home.
In the 2009-2011 period, the Standard & Poor's 500-stock index soared 34 percent -- and has since risen an additional 26 percent -- while the S&P/Case-Shiller home price index fell 5 percent, continuing a steep slide that began with the 2006 housing-market crash.
Housing prices began slowly rebounding in the past year but remain 29 percent less than their 2006 peak.
Overall, U.S. household wealth rose $5 trillion, or 14 percent, during the two years, to $40.2 trillion in 2011 from $35.2 trillion in 2009, Pew said.
The report focused on distinguishing the top 7 percent of households from the rest, rather than a different high-wealth category, because of "the limits of the tabulations published by the Census Bureau," study authors Richard Fry and Paul Taylor said in the report.