The number of Californians without medical insurance grew from about 5.2 million to almost 6 million during a recent three-year period, and almost 90% of the uninsured are people who work for a living and their families, according to a UCLA study released Tuesday.

The report by the UCLA School of Public Health analyzes the swelling ranks of medically uninsured Californians at a time of mounting public concern about the problem but little legislative momentum toward a solution. Based on 1990 Census reports, the UCLA study concludes that 22.5% of the state's residents under 65 are medically uninsured--compared to 16.5% nationally.

The report says that 22% of Orange County residents under age 65 have no health insurance--305,605 people. Though it did not offer detailed breakdowns by county, the study's principal author, E. Richard Brown, said that Orange County ranks second among California counties for residents under age 65 who lack health insurance and fifth among the 30 largest metropolitan areas in the United States for non-elderly residents without health insurance.

Almost half the uninsured, or 2.7 million people, live in Los Angeles County, which has the highest percentage of medically uninsured people of any major metropolitan area in the nation. The percentage jumped from 26.7% in 1986 to 33% in 1989.

About 65% of the medically uninsured in the state are full-time jobholders and their families; another 22% are self-employed or part-time workers and their families.

Children make up nearly one-third of the uninsured. Adults 18 to 29 account for 36%.

Of all ethnic groups, Latinos are most likely to be uninsured. Researchers stated that "a shocking 42%" of the non-elderly Latino population lacks health coverage--mainly because they are employed in small businesses and in fields that typically do not provide health insurance to employees.

"The root of the problem," said UCLA professor E. Richard Brown, "is the structure of our state economy, which directs many workers into low-wage jobs in small, often marginal firms with no health care benefits."

The health-care needs of Americans 65 and over are covered by the federal Medicare program, and many poor people are covered by the Medicaid program, known as Medi-Cal in California.

But the vast majority of Americans traditionally have obtained health insurance through their workplace, as a fringe benefit of employment.