Only one fifth or fewer of the female sexual partners of HIV-1-infected men with hemophilia become infected. The risk factors associated with heterosexual transmission of HIV-1 are not well understood. To investigate the hypothesis that HIV-1 viral load may be related to heterosexual HIV-1 transmission, we measured HIV-1 RNA by polymerase chain reaction (PCR) in frozen samples from 39 men with hemophilia and HIV-1 infection obtained between 20 and 62 months after HIV-1 seroconversion, during at least a 6-month relationship with a female sexual partner. The median time from the hemophilic viral load determination to the estimated date of transmission to the female partner was 9 months (range, 4-41 months). The proportion of HIV-positive hemophilic men with >100,000 HIV RNA copies/ml was significantly higher in transmitters (TR) (3 of 5 [60%]), than in nontransmitters (NTR) (3 of 34 [9%]; p = 0.027). There were no differences between TR and NTR in age at seroconversion (32.4 years each), in time from seroconversion to AIDS (67 versus 79 months), in mean CD4 number (245/microl] versus 260/microl); nor in the proportion who developed AIDS (4 of 5 [80%] versus 24 of 34 [71%]). These findings appear to suggest that high HIV viral load in HIV-infected hemophilic men increases the risk of HIV transmission to heterosexual partners. Viral load determinations may be helpful in counseling hemophilic couples regarding transmission to female partners.