AIDS-related Kaposi's sarcoma (KS) has a better prognosis than other presentations of AIDS. It is more frequent among homo-bisexual men than in other risk groups. This has raised the possibility of specific etiological agent(s) or co-factor(s) which induce its development. However, this suggestion is confusing since it implies that exposure to such agent(s) or co-factor(s) would result in an improved prognosis. Recent virological studies indicate that the HIV itself could be responsible for the induction or growth of KS. Since HIV antigenemia and the immunosuppression are lower among KS cases than among those with opportunistic infections (OI), we could expect a lower level of exposure to AIDS-related factors in KS cases as compared to OI cases. To investigate this possibility, we compared 25 cases with Kaposi's sarcoma alone (KS) with 25 cases having OI without KS, among homo-bisexual men. The KS cases were more likely than OI cases to have a higher educational level. They were less likely to have inhaled nitrites, to have had repeated syphilis and repeated gonorrhoea, and to be promiscuous. In the multivariate analysis, the factors which best discriminated the groups were nitrite inhalations, history of repeated syphilis, and anonymous promiscuity ("one-night stands"). Our study does not support the hypothesized association between nitrite inhalations and Kaposi's sarcoma. We found that cases with opportunistic infections - that is, with a poor prognosis - had been exposed to AIDS-related risk factors (e.g. history of venereal diseases, nitrite inhalations, promiscuity) at higher levels than cases with Kaposi's sarcoma alone.(ABSTRACT TRUNCATED AT 250 WORDS)