QUESTION: Post-exposure prophylaxis (PEP) is effective in preventing HIV infection after professional exposure. PEP is also recommended after inadvertent sexual exposure if two conditions are met: the source person is known to be HIV infected, and exposure to this person occurred only once or is accidental. The objective of this study is to describe the circumstances of sexual exposure and determine how frequently these conditions were present among patients diagnosed during primary HIV infection. METHODS: Physicians of 35 patients diagnosed with primary HIV infection provided detailed information on the circumstances of infection as well as on the patients and source persons. RESULTS: Most patients were homosexual (71%), and 91% were male. Only one patient (3%) had a single exposure with a known HIV-infected person and was therefore eligible for PEP. Eight patients (23%) who did not know the HIV status of their partner would have been eligible considering single exposure as a sufficient criterion for PEP. Oro-genital contact appears to account for transmission in four instances. Eleven persons (31%) were infected after sexual contact with their stable partner, of whom 7 did not known his/her HIV status. Twelve patients (34%) were infected after multiple unprotected sexual contact with unknown partners. CONCLUSION: The direct impact of PEP in terms of HIV infections prevented is likely to be small. PEP will not make up for the failure of other prevention methods. It may, however, contribute to the disclosure and the discussion of risk situations and help physicians provide individual counselling taking into account the precise context of risk behavior.