Acquired immune deficiency syndrome (AIDS) has been etiologically linked with the human T-cell lymphotropic virus HTLV-III. In a study on the prevalence of antibodies to this virus in Switzerland, sera from 941 individuals were collected in 5 major urban areas (Basel, Berne, Geneva, Lausanne and Zurich) in 1983 and 1984. All sera were tested by ELISA and the majority also by Western blot. We found an antibody prevalence of 100% among 22 cases of AIDS, of 94% among 48 cases of AIDS-related complex (ARC), and of 57% among 14 homosexual contacts of AIDS patients. Among 55 sera collected from i.v. drug addicts in 1984, 53% were positive, whereas the rate had been 36% among 103 sera collected in 1983. The rate was 19% among 227 sera collected from asymptomatic active homosexuals in 1984, against 10% among 40 sera collected in 1983. 8% positives were found among 98 patients with various types of viral hepatitis. In addition, 4% of 84 sera of individuals from Equatorial Africa were positive. No positives were found among 15 sera of individuals from other African regions, among 32 sera of persons from various regions of Asia, and among 203 Swiss blood donors. These results demonstrate the high and increasing rate of HTLV-III infection among groups at risk for AIDS, and suggest that HTLV-III-related diseases will be a serious problem in the years ahead.