We studied 100 unselected parenteral drug abusers for infection with hepatitis C, B, A and D virus (HCV, HBV, HAV and HDV). Seventy-six percent had serological evidence of HCV infection. 12% were positive for HBsAg and at least one marker of HBV infection was present in 69%. These results were significantly higher than in a matched control population. Compared to controls, the prevalence of anti-HAV (65%) was not significantly increased in drug addicts. Of the anti-HCV-positive drug addicts, 80.3% had at least one marker of HBV infection compared to 33.3% of anti-HCV-negative cases (p less than 0.001). No such correlation was found between the prevalence of HCV or HBV infection markers and the presence of anti-HAV. Antibodies against HDV were detected in 16 (16%) of the samples from drug addicts. No significant association was found between antibodies to HCV and gender, age and duration of drug abuse. The risk of HBV infection increased significantly with years of drug abuse but was not associated with age and sex. The presence of anti-HAV was related to age only. Sixteen (16%) of the subjects were definitely positive for anti-HIV-1, but at the time of the study they were asymptomatic. No significant association was found between the presence of anti-HIV and the prevalence of serological markers of HBV, HCV, HAV and HDV infection.