The Answer:

According to the Jewish Virtual Library, there were an estimated 6,155,000 Jewish people in the United States in 2001, which is more than there were in any other country. (Israel came in second.) This represents approximately 2.2% of the total U.S. population.

The same site has a chart of the state-by-state breakdown of the Jewish population, revealing that New York had both the highest number (1,657,000) and highest percentage (8.7%) of Jews in the country, by a wide margin.

For more information, see History of Judaism and Religion.

—The Fact Monster