In Denmark, AIDS has primarily affected men with homosexual behavior. In order to follow the occurrence and development in this group, the sero-epidemiological investigations which focus on the underlying HIV epidemic are reviewed. HIV was introduced in Denmark towards the end of the nineteen-seventies and spread rapidly in the beginning of the nineteen-eighties among promiscuous homosexual men in Copenhagen to a level in which 1/4-1/3 were found to be infected in small selected materials. From the middle of the nineteen-eighties, decreasing tendencies were observed in the numbers of homosexual and bisexual men in whom antibodies to HIV were demonstrated for the first time. Epidemiological evidence suggests that the rate of spread of HIV among homosexual men slowed down in the second half of the nineteen-eighties. In 1985, it was estimated that at least 6,000 homosexual men were infected with HIV. No investigations justify new increased estimates of the numbers of HIV-infected homo- and bisexual men in Denmark. Epidemiological data demonstrate clearly that unprotected receptive anal coitus is the main route of infection for homosexually transmitted HIV. The risk of infection appears to be much lower for the active partner in anal coitus with an HIV-infected man. No epidemiological investigations have demonstrated oral sex as the route of infection for HIV among men although this does not exclude the possibility of HIV-infection in occasional cases of oral sex.