In order to study the influence of cigarette smoking and drinking of alcohol on the progression to acquired immunodeficiency syndrome (AIDS), eighty HIV infected homosexual men were included in a prospective study from the date of diagnosed HIV seropositivity. Two men were lost to follow-up. The mean follow-up time was 62 months. By the end of the follow-up period 26 out of 78 subjects (33%) were diagnosed with AIDS. When controlling for age, year of HIV diagnosis, number of male lifetime partners and frequency of receptive anal intercourse, the adjusted relative risk of being diagnosed with AIDS for the group smoking 1-20 cigarettes daily was 0.4 (0.2-1.2, 95% confidence interval) and 1.1 (0.4-2.7, 95% confidence interval) for the group smoking more than 20 cigarettes daily, as opposed to the non-smoking group. The adjusted relative risk of progression to AIDS for daily alcohol drinkers as opposed to less frequent drinkers was 0.8 (0.3-2.2, 95% confidence interval). The adjusted relative risks of receptive anal intercourse often or usually and more than 500 lifetime male partners were 2.2 and 2.0, respectively. These estimates, however, were not significantly above 1.0. The lack of positive association between cigarette smoking, drinking of alcohol and progression to AIDS found in this study as well as in other studies, may have implication for the understanding of the pathogenesis of the HIV disease and for counselling HIV infected subjects.