Of 686 hemophiliacs who are being treated at our institution, 402 (59%) are HIV-seropositive. Onehundredseventy-eight heterosexual partners of HIV-infected hemophiliacs have been serologically examined; 19 (11%) are HIV-positive. So far none of the seropositive partners suffers from ARC or AIDS. The rate of heterosexual transmission of HIV is statistically significantly correlated with the CD4⁺ count of the HIV-infected index patient. No such correlation was found with the index patient's clinical stage or the isolation of HIV from the index patient's blood. Of 39 seronegative female partners who were investigated clinically and immunologically, 17 showed pathologically increased numbers of CD8⁺ counts. In one case, HIV was transmitted from a female patient with von Willebrand's disease to her husband. As compared to other groups at risk for AIDS, the rate of heterosexual HIV transmission is comparatively low in hemophiliacs. The exact reason for this difference is not yet known. The relevance of the immunopathological findings in seronegative sexual partners of hemophiliacs also remains to be determined.