The transmission of the human immunodeficiency virus (HIV) was studied in 647 subjects who presented no apparent risk factors for the infection other than having had promiscuous heterosexual relations, heterosexual relations with people with an elevated risk of infection, or heterosexual relations with people infected by human immunodeficiency virus. Thirty subjects were found to be seropositive for anti-human immunodeficiency virus antibodies. The elevated risk factors included being the habitual partner of a person at risk of infection or of a person who was infected by human immunodeficiency virus, or being the partner of a patient with acquired immunodeficiency syndrome. The transmission of the virus was verified in 13 of 284 subjects (4.57%) who had had heterosexual intercourse three or more times with persons at risk and in 16 of 101 subjects (15.84%) who had had heterosexual intercourse three or more times with persons who were seropositive for human immunodeficiency virus antibodies. No significant correlation between human immunodeficiency virus infection and a history of sexually transmitted infections, nor between human immunodeficiency virus infection and female subjects was found. These data suggest that the epidemic of acquired immunodeficiency syndrome can also spread through heterosexual relations, even if the possibility of becoming infected in this way seems at the moment limited to particular risk behaviors.