To identify risk factors for A.I.D.S. among homosexual men in France, a high-risk group in a moderate-incidence area, we undertook a case-control study in Paris and its suburbs. Fifty-three cases were compared to 99 controls from a venereal disease clinic and 79 controls from the staff of an amateur gay broadcasting station. In our univariate analysis, cases were more likely than controls to belong to upper socio-economical classes, to have used local corticosteroids, to have regularly inhaled nitrites, to report history of syphilis and of herpes infections, to have higher level of promiscuity with occasional partners ("one night stands"), and to have had sexual encounters in the U.S.A. In the multivariate analysis, history of syphilis and promiscuity with occasional partners appeared to be the main risk factors, as well as the use of local corticosteroids, especially during the prodrome period. The correlation between promiscuity and disease risk was weak in our study when we did not discriminate between occasional and regular sex partners, contrary to that found in epidemiological studies of A.I.D.S. and H.I.V.-seropositivity in high-incidence areas. In association with the history of syphilis, the development of the syndrome in our moderate-incidence country is more correlated to the contact with "one-night-stand" partners rather than to the total number of the sex partners. The public health policies in our area should be adapted to these findings. Otherwise they may be less efficient than hoped.