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More than 10 percent of all American Jews reside in three counties in south Florida.

The Jewish population in those counties — Palm Beach, Broward and Miami-Dade — now stands at 644,000. That's a 30 percent increase over the past 20 years.

These figures come from a new study, commissioned by the Jewish Federation of Palm Beach County, that details synagogue membership, adherence to Jewish rituals and the intermarriage rate.

"The three-county south Florida area is the second largest concentration of Jews in the country after the New York metropolitan area," said Ira Sheskin, a professor of geography at the University of Miami and the study's author.

Sheskin, a nationally recognized Jewish demographer, said the local community has been successful in dealing with the needs of a growing elderly population.

The southern areas of Palm Beach County — Delray Beach and Boca Raton — boasts the oldest Jewish population in the nation: 69 percent are age 65 and over.

What is significant about these numbers is that most children of older Jewish adults in south Florida have not followed their parents to the Sunshine State. Adult children can often provide a support system in times of medical or economic crises.

"Most of the children of older people have not moved to south Florida with them, which creates problems and challenges for those involved in social services and other helping programs," Sheskin said.

Released in the fall, the study found that there are 101,568 people in Jewish households living in northern and central Palm Beach County, making the area the 12th-largest Jewish community in America.

In 1987, the last time a full demographic study was undertaken by the local Jewish federation, there were 76,204 people in Jewish households living in the same area.

This incredible burst of growth amounted to 33 percent in 12 years. The north-central Palm Beach County Jewish community is now larger than those in Cleveland, Detroit and Milwaukee.

If all of Palm Beach County is counted, the total becomes 230,000 people in Jewish households, and the entire county becomes the sixth-largest Jewish area in the United States, making it larger than Jewish populations in the San Francisco Bay Area, Boston, Philadelphia and Washington.

Some other key findings:

*The median income is $59,000, which includes 1,000 families living below the poverty line. Twenty-eight percent have a net worth of more than $500,000.

*Two percent are Orthodox, 34 percent Reform, 39 percent Conservative. The remaining 25 percent identified themselves as "just Jewish."

*Palm Beach County has the nation's second highest rate of Jewish households with members who have visited Israel — 57 percent. The southern area of Palm Beach County has the highest rate at 61 percent.

*Palm Beach County has a very low intermarriage rate, at 11 percent.

*A significant number of local residents participate in Jewish rituals and customs in their homes. For example, 89 percent affix a mezuzah and 87 percent attend a seder.

*Ninety-three percent of households pay membership dues to a Jewish organization or synagogue or donate to Jewish charities. Some 37 percent of those surveyed belong to a synagogue somewhere in the United States, including about 19 percent who belong to a synagogue in Palm Beach County.

Just over 1,000 Jewish households were surveyed. They included some non-Jewish spouses and children residing with Jews. The survey's margin of error is plus or minus 3 percent.