Risk factors in the development of adverse reactions in HIV-1-infected patients treated with highly active antiretroviral therapy (HAART) containing protease inhibitors are poorly understood. To identify predictors of protease inhibitor-associated adverse events, we are conducting a prospective, cohort, multicenter study on HIV-positive patients starting treatment with at least one protease inhibitor. Rate ratios (RR) of adverse events were calculated, and logistic regression was used to adjust simultaneously for the potentially confounding effects of selected variables, according to the Cox model. A total of 1477 patients have been enrolled up to April 2000, having an average age of 37.1 years (SD +/- 8.1); 1066 (72.2%) were male. Where risk factors for HIV infection are concerned, the distribution was as follows: 48.1% intravenous drug users, 31.6% heterosexual contacts, 16.2% homosexual males and 0.7% blood transfusion. Average CD4+ lymphocyte count at enrollment was 265 cells/mmc (SD +/- 201). Average follow-up time is equal to 17.8 months (range 1-32). The risk of developing adverse reactions is significantly increased in female patients, older patients, hemophiliac subjects and in subjects with hepatitis. Patients treated with ritonavir, the association ritonavir-saquinavir HGC, stavudine and efavirenz have significantly increased incidence of adverse reactions in PI-containing regimens; conversely, saquinavir HGC, zidovudine and lamivudine use was associated with a lower risk of developing adverse reactions.