The risk of human immunodeficiency virus (HIV) transmission was studied by interviewing and testing the serum of heterosexual contacts and casual family contacts of adults with transfusion-associated HIV infections. Two (8%) of 25 husbands and ten (18%) of 55 wives who had had sexual contact with infected spouses were seropositive for HIV. Compared with seronegative wives, the seropositive wives were older (median ages, 54 and 62 years; P = .08) and actually reported somewhat fewer sexual contacts with their infected husbands (means, 156 and 82; P greater than .1). There was no difference in the types of sexual contact or methods of contraception of the seropositive and seronegative spouses. There was no evidence of HIV transmission to the 63 other family members. Although most husbands and wives remained uninfected despite repeated sexual contact without protection, some acquired infection after only a few contacts. This is consistent with an as yet unexplained biologic variation in transmissibility or susceptibility.