An epidemic of hepatitis A virus (HAV) among intravenous drug abusers in Oslo involved 144 serologically confirmed cases. Another 26 patients (non-drug abusers), of whom 14 were derived from a single nosocomial outbreak, were associated with the epidemic. Sequencing of the VP1/P2A junction revealed that viruses associated with the epidemic were completely identical, whereas other HAV samples collected during the same period differed by up to 10%. HAV was detected in the serum of 48 of 100 patients by a nested PCR. Viremia was observed as early as 25 days before the onset of clinical hepatitis, and up to 30 days after. The large number of patients within the drug abuser group, and the few secondary cases, raised the question of whether the virus could be transmitted by the use of needles. To establish whether viral contamination of drugs did contribute appreciably to maintaining the epidemic, we examined heroin and amphetamine confiscated during the period, using immunomagnetic separation coupled to nested PCR, but failed to detect any virus. Antibodies against hepatitis B virus and hepatitis C virus were common among the HAV infected drug abusers (43% and 81%, respectively), suggesting widespread sharing of needles. This observation and the large number of patients with a demonstrable viremia suggest that needle sharing may contribute to the dissemination of HAV.