Eighteen transfusion recipients infected with human immunodeficiency virus type 1 (HIV-1) were followed prospectively with their 19 long-term sexual partners from 1986 to 1993 in California, Florida, and New York. Follow-up included clinical, behavioral, immunologic, serologic, and virologic evaluations. Two partners were already infected when seen 18 and 34 months after sexual contact began following the infectious transfusion. Four of 17 initially seronegative partners seroconverted during 23 person-years of observation. The recipient's clinical status, mononuclear cell subset variations, and time trend in CD4+ counts had no association with transmission. Individual plasma HIV-1 ribonucleic acid (RNA) loads were stable during observation, and sexual transmission was not attributable to an upward trend or transient burst in viremia. However, recipients who transmitted HIV-1 to their sexual partners had higher mean viral RNA levels than did nontransmitting recipients (4.3 vs. 3.6 log10 copies/ml; p = 0.05). Although this series was small, the prospective observations suggest that viral load was the only characteristic in the recipient that contributed to heterosexual infectiousness.