Sera from 260 men from Denmark and elsewhere attending two Copenhagen sauna clubs for homosexual men during nine months of 1982-1983 were investigated for markers for syphilis, hepatitis A and B, and human immunodeficiency virus (HIV). Five per cent (12 men) had active syphilis, and another 35% (92) had a history of and/or serologic markers for syphilis. Ninety-four men (36%) were positive for antibodies to hepatitis A virus, ten (4%) were positive for hepatitis B surface antigen (HBsAg), and 153 (59%) were positive for antibodies to HBsAg. Antibodies to HIV were found in 45 (20%) of the 220 men investigated for this marker. Markers for hepatitis A and B and for syphilis were more frequent in the HIV antibody-positive individuals, but the association was significant only for markers for hepatitis B (relative risk = 2.0). Thus STD markers had little predictive value for seropositivity for antibodies to HIV. Among 37 men investigated more than once, a seroconversion rate of 3% per month for antibodies to HIV was found, but this estimate must be taken with reservation. The rate of seropositivity for antibodies to HIV among men from Denmark was 23%, and three (8%) of the 40 HIV-positive Danish men developed the acquired immunodeficiency syndrome (AIDS) during the four years following the initial investigation. This study shows that by 1982-1983 HIV had spread considerably in the Danish high-risk group, although there were only seven reported cases of AIDS in the country at that time.