OBJECTIVE: To characterize the circumstances underlying the epidemic of AIDS associated with blood transfusion in Mexico and to explore the possible mechanisms for its dissemination. METHODS: A retrospective analysis comparing the total number of AIDS cases and transfusion-associated AIDS cases and the male:female ratio reported in Mexico and the U.S. from 1981 to 1996 was done. We analyzed the relationship between the location of plasmapheresis banks and the geographic distribution of transfusion-associated AIDS cases and AIDS cases among paid donors in order to assess the possible role of plasmapheresis banks in its dissemination. RESULTS: The proportion of transfusion-associated AIDS in the total number of cases was significantly higher in Mexico than in the U.S. from 1987 through 1996 (p < .0001). A rapid drop in the male:female ratio of AIDS was observed in Mexico but not in the U.S. coinciding with a growing number of transfusion associated cases; transfusion has been the main risk factor for AIDS in women in our country. In 1986, seven States and Mexico City had plasmapheresis banks: they reported 90% of the cases associated to paid donation and 75% of those associated to transfusion, despite the fact that commercial blood banks without plasmapheresis facilities existed in 23 of the other 24 States. CONCLUSION: There was a difference on the frequency of transfusion associated AIDS between Mexico and the U.S. which reached epidemic proportions in Mexico. We believe that plasmapheresis banks played a major role in the dissemination of the infection in Mexico as paid donors provided a third of the blood used in Mexico in 1986. These findings highlight important implications for the prevention of AIDS in developing countries where commercial plasmapheresis practices are still in operation.