39 persons with an incidentally discovered seroconversion from HIV antibody negative (Ab-) to antibody positive (Ab+) state as measured by an enzyme-linked immunosorbent assay (ELISA) were investigated for the presence of (1) HIV antigen (Ag) and (2) immunoblotting test (IBT) Ab in serum samples collected within the year before seroconversion. 13 (33%) of the patients were HIV Ag+ at some time before seroconversion. However, the collection of samples was not done systematically and the samples from patients who had at least 1 sample collected within 3 months before seroconversion were thus compiled separately. This group consisted of 58 samples from 19 patients and among these none were HIV Ag+ earlier than 11 weeks before seroconversion, but the prevalence of HIV Ag+ samples was rising towards seroconversion and 10 patients (53%, 95% confidence limits: 29-76%) became HIV Ag+ in this 11-week period. Further, among all patients 13 (33%) were IBT Ab+ 4-50 days (median: 14 days) before seroconversion. Finally, among 18 patients with signs and symptoms consistent with an acute HIV infection 10 were HIV Ag+, as opposed to 4 HIV Ag+ patients among 21 without symptoms (p = 0.041).