P. carinii pneumonia affects more than 80% of patients with AIDS in developed countries. In Mexico this complication has been described in 12 to 23% of patients with AIDS, and it has been proposed that competitive disease such as TB and the low life expectancy of AIDS patients in Mexico are the possible explanations of this low percentage. However, in Mexico the frequency with which the population at risk of contracting AIDS is exposed to P. carinii is unknown. Therefore, we looked for P. carinii antibodies in a group of blood samples obtained in an AIDS detection center in Mexico City. A Western blot analysis was performed on 157 blood samples from homosexual and bisexual men and on 21 from female partners of HIV positive men. Positive results were obtained for 76.9% of the samples. No difference was observed in the frequency of P. carinii antibodies between HIV positive and negative individuals. Also, no differences by sex or age were found. Prevalence of P. carinii antibodies in this group is 76.9%, similar to that found in a comparable group in the U.S.