Seven males of 132 (5%) and 9 females of 85 (10%) out of 217 heterosexual individuals without addiction to drugs or previous blood transfusion who attended a STD clinic in Madrid because they had high risk sexual partners were found to have antiHIV antibodies by ELISA and immunofluorescence (CNMVIS). However, when variables such as sex, promiscuity and sexual habits were correlated with HIV infection, significant differences were not found between seropositive and seronegative individuals (kappa 2, p greater than 0.05). Among the factors statistically linked with the infection (kappa 2, p less than 0.05) there were the previous STD and the sustained sexual intercourse with proven HIV seropositive individuals: there 8 infected individuals out of 40 with such type of partners (relative risk 4.4). On the other hand, the partners of individuals with unknown serological findings had the following results: 4 seropositive individuals out of 71 partners of parenteral drug abusers, 3 of 82 users of prostitutes, and 1 infected individual out of the 24 remaining ones (partners of bisexual individuals or combinations) (relative risk of each of them less than 1). These results show the importance of heterosexual, not mediated by blood, HIV transmission in couples of high risk persons, and they suggest that the infection is highly likely if sexual intercourse is maintained with infected individuals irrespective of their sex.