Wealth in the United States became much more concentrated in the two decades between 1963 and 1983, as the share owned by the top 0.5 percent of American households rose from 25 percent to 35 percent, according to a congressional study released yesterday.

The real value of household assets less debt owed on them nearly tripled, rising from $3.9 trillion in 1963 to $10.6 trillion after adjustment for inflation, according to the study prepared by the Democratic staff of the Joint Economic Committee.

The basic data was drawn from a survey of consumer finances and wealth conducted in 1983 by the Survey Research Center of the University of Michigan for the Federal Reserve and several other federal agencies.

The JEC study provides no explanation for the dramatic increase in the concentration of wealth, which reversed a long-term downward trend in the share of wealth held by the richest 0.5 percent of all households.

"We don't know for sure why this happened," said Scott Lilly, executive director of the JEC. "But we do feel comfortable saying this is the best data there is. It is a remarkable reversal in a 40-year trend in which the concentration of wealth declined."

The richest 0.5 percent of households had net wealth of $2.5 million or more, the study said. The next lower 0.5 percent had net wealth of between about $1.4 million and $2.5 million. The next lower 9 percent of all households held assets worth between roughly $200,000 and $1.4 million.

Together, these 10 percent of households owned more than 70 percent of the nation's wealth not held by governments or private organizations, such as nonprofit groups.

The average growth in real wealth for all households during the 20-year period was $55,600. The average wealth-holding for the lower 90 percent of households rose from $27,400 to $39,600. The richest 0.5 percent increased their average wealth from $3.59 million to $8.85 million, the study said.

The household figures did not include the value of some consumer assets, such as household furniture. The estimated $1.3 billion worth of assets in employer-provided pension funds also were excluded because of the difficulty of linking ownership to particular households.

During the 20-year period, the JEC study said total net wealth rose by $6.7 billion. Total wealth, ignoring debt owed on it, rose from $4.6 trillion to $12.1 trillion.

The largest increase in asset values was for real estate. The value of owner-occupied homes rose from $1.5 trillion in 1963 to nearly $3.6 trillion in 1983. The value of other real estate rose even faster, from about $400 billion to about $1.8 trillion.

The other extremely large gain in aggregate value was in the form of net business assets, not including corporate stock. The value of such assets rose from about $900 billion to nearly $3.3 trillion, the study said.

All of the figures cited in the analysis other than those for 1963 and 1983 were based on an extrapolation of federal estate-tax returns to the general population. That approach showed the top 0.5 percent of households owning about 32 percent of total household wealth in 1929.

The 1983 data are based on a survey of 3,824 randomly chosen households plus interviews with an additional 438 high-income families. The 1963 data came from a similar survey.

A recent Census Bureau study of wealth held in 1984 did not include such reporting from high-income families, although its total sample of 26,000 households was much larger. The Census Bureau described its wealth estimates as "comparable" to the 1983 Federal Reserve estimates. Neither the Census Bureau nor the Fed made estimates of the concentration of wealth in the same fashion as the JEC.

While the amount of wealth owned by the top 0.5 percent of households was rising sharply, the share of every other group shown by the JEC was declining.

The next 0.5 percent in terms of wealth saw its share decline from 7.4 percent to 6.7 percent. The share of the next 9 percent of households fell from 32.3 percent to 29.9 percent. That of the remaining 90 percent dropped from 34.9 percent to 28.2 percent.