Data from the National AIDS Behavioral Survey were used to examine the social distribution of extramarital sex and risk for human immunodeficiency virus (HIV) infection among married individuals in the United States. Of 1686 married respondents living across the United States, 2.2% reported extramarital sex; of 3827 married respondents living in 23 urban areas with large Hispanic or African-American populations, 2.5% reported having sexual partners outside marriage. The data indicate that the correlates of extramarital sex varied by race/ethnicity. Low levels of condom use were found among people reporting extramarital sex (8% to 19% consistent users).