Data were collected as a supplement to the Los Angeles Epidemiologic Catchment Area project, one of five field sites of a National Institute of Mental Health-initiated program. The authors used a two-stage probability sampling technique to interview 3,132 Los Angeles residents of two mental health catchment areas during the period January 1983-August 1984. Hispanics and non-Hispanic whites were about equally represented in the sample, as were males and females. Respondents were asked, "In your lifetime has anyone ever tried to pressure or force you to have sexual contact? By sexual contact I mean their touching your sexual parts, your touching their sexual parts, or sexual intercourse?" Persons who reported an assault were asked additional questions, including information about the most recent assault. Lifetime prevalence of sexual assault during adulthood (at or after age 16 years) was estimated at 10.5% for the entire sample. Women, non-Hispanic whites, and young people (less than age 40 years) reported higher rates of sexual assault than men, Hispanics, and older people. Highest rates were reported by young non-Hispanic white women with some college education (26.3%). In the most recent sexual assault, three-fourths of respondents knew their assailant, over half experienced harm or the threat of harm, and half experienced sexual contact including but not limited to intercourse.