The prevalence of hepatitis delta virus (HDV) infection was studied in 140 unselected intravenous drug addicts whose sera were drawn between 1985 and 1986 and in 100 addicts from whom sera were obtained between 1988 and 1989. It was observed that 1.8% of those positive for hepatitis B virus (HBV) markers from the earlier period and 23.2% of those from the later period had detectable anti-delta antibodies. Among drug addicts referred with acute or chronic HBV infection, the first evidence of HDV infection was found in 1986. We conclude that HDV infection was introduced into the Warsaw drug community in the mid 1980s.