This observational study compared the probability of developing the first opportunistic infection among AIDS adult patients attending the "Programa SIDA de San Juan" who received either indinavir plus zidovudine and lamivudine (n = 45) or ritonavir plus zidovudine and lamivudine (n = 16) between August 1, 1996 and July 31, 1997. No significant differences were observed with respect to appearance of an opportunistic infection, increase in CD4 levels and decrease in viral load between both groups during the study period. However, an increased probability of being free of opportunistic infection after 10 months was observed for the indinavir group (p > 0.05). In addition, the probability of changing or interrupting prescribed therapy was 2 times higher for patients under ritonavir (p < 0.05). These results suggest the need to confirm these findings in a larger group of patients in a controlled clinical trial and to assess the short-term and long-term effects of both therapies among Puerto Rican AIDS patients.