OBJECTIVE Human immunodeficiency virus type 1 (HIV) is cultured more often from seminal cells than seminal plasma. Because vasectomy causes dramatic reductions in seminal cells and also eliminates secretions from proximal sites in the male reproductive tract, vasectomy may change the potential infectiousness of semen. METHODS We used polymerase chain reaction (PCR) assays to measure HIV ribonucleic acid (RNA) in seminal plasma and HIV deoxyribonucleic acid (DNA) in seminal cells from 46 asymptomatic, seropositive men before and after vasectomy. RESULTS HIV RNA levels in semen correlated only weakly with blood levels (r = 0.22, p = 0.03). Of 183 semen specimens assayed for cell-free HIV RNA and proviral DNA 37 (20%) were positive for HIV RNA only, 41 (22%) were positive for HIV DNA only, and 18 (10%) were positive for RNA and DNA. Thus, detection of HIV RNA in seminal plasma was not associated with detection of HIV DNA in seminal cells. HIV RNA was present in 23 of 82 specimens (28%) (mean 2.87 log copies/ml.) before vasectomy and in 38 of 121 specimens (31%) after vasectomy (mean 2.81 log copies/ml.). CONCLUSIONS These findings suggest that direct measurement of HIV levels in semen is necessary to assess the potential for sexual transmission, most cell-free HIV in seminal plasma arises distal to the vas deferens, and vasectomy may have minimal impact on the infectiousness of HIV seropositive men on sexual partners.