612526 blood donations collected by the Central Laboratory of the Swiss Red Cross Blood Transfusion Service between July 1985 and June 1987 were routinely tested for antibodies to HIV.96 donations (82 men, 14 women were anti-HIV positive (0.157%, 1 of 6369 donations). The prevalence of anti-HIV positive donations was higher in men (0.172%, 1/5814) than in women (0.104%, 1/9615). Donations collected in military units were markedly more frequently seropositive (0.570%, 1/1754 in refresher courses, 0.261%, 1/3831 in recruit training). Excluding military donations, donations from men (0.077%) were less frequently HIV-seropositive than donations from women (0.104%). The prevalence of seropositive donations was 4.4 times higher in new donors than in repeat donors. Regarding age distribution, the peak of seropositive donations was observed, as expected, in the third decade. Donations from French speaking Swiss had a clearly higher HIV prevalence than those of German speaking Swiss. As a result of continuous donor information and selection, the number of anti-HIV positive results decreased during the 4 half years covered by our study. It was lowest at the end of the observation period (0.098% or 1/10200 between January and June 1987). Doubtful positive results were recorded chiefly among women, at a rate increasing with age. The majority of such results are presumably due to antibodies with other specificities and must therefore be considered false positive.