To test the hypothesis that safer sex procedures are less consistently observed by persons under the influence of alcohol, data from the Swiss human immunodeficiency virus (HIV) Prevention Study (HIPS) were evaluated. HIPS is a large prospective cohort study involving 724 HIV-negative and mainly heterosexual subjects who entertain casual sexual contacts. Of the 724 participants, 36% reported that they had had sex while under the influence of alcohol. Of this group, 31% indicated that safer sex procedures were neglected owing to the influence of alcohol. No significant differences with regard to unprotected sexual intercourse were found between subjects who combine sex and alcohol and those who do not. The same was found to be true among subjects with different levels of general alcohol consumption. However, a significant correlation was found between the intensity of alcohol consumption (i.e., the quantity of alcohol intake per sitting) and the incidence of unprotected sexual intercourse. These findings show that the relationship between alcohol consumption and safer sex is complex; they also emphasize the need for preventive efforts to reinforce safer sexual behavior, for example through individual counseling of persons at risk for HIV-infection.