The aim of our study is to compare the tolerability of zidovudine/lamivudine/indinavir when used in post-exposure prophylaxis (PEP) subjects and in HIV-infected patients. HIV-negative patients were enrolled as part of the surveillance protocol for professional exposure at the Luigi Sacco Hospital in Milan. HIV-positive patients were selected among all subjects undergoing treatment with zidovudine/lamivudine/indinavir from the CISAI cohort, an Italian cohort for the evaluation of adverse reactions to HAART. In both studies patients were followed prospectively and the severity of the reactions was evaluated using the AIDS Clinical Trial Group adverse experience grading scales. Up to September 1999, 37 HIV-seronegative subjects had undergone treatment with zidovudine/ lamivudine/indinavir. From a total of 1207 patients belonging to the CISAI cohort, 199 were identified as being treated with the same regimen. The frequency of adverse events in the PEP subjects was 70.3% compared to 11.1% for HIV-infected patients. In the first group, adverse events caused treatment interruption in 21 subjects (56.7%) versus 14 patients (7%) among the HIV-infected group. Only one case of a severe event (grade 3-4) was observed in the prophylaxis group against 12 in the treatment group. Our study shows that treatment interruption is eight times higher in HIV-negative subjects compared to HIV-seropositive patients, and that the incidence of adverse events is approximately six times higher, though such events, are for the most part, not severe.