Responsiveness was assessed to a programme of vaccination of hepatitis B vaccine in a cohort of 197 intravenous drug addicts (mean age, 23.7 years) and their antibody response was compared with that of 271 healthy controls (mean age, 24.2 years). All participants were seronegative for hepatitis B surface antigen (HBsAg) and antibody to HBsAg (anti-HBs). The vaccination schedule consisted of three intramuscular injections (deltoid area) at months 0, 1 and 2. Although 70% of parenteral drug abusers received the three doses of vaccination, only 43.6% were evaluable for immune response. Fifty-eight per cent of heroin addicts and 80% of controls had evidence of anti-HBs seroconversion at 1 month after vaccination (χ2 = 15.52, p < 0.001). Geometric mean antibody titres were also significantly higher in controls (69.1 IU l-1; confidence interval 95%, 56.83 and 84.04) than in parenteral drug abusers (18.2 IU l-1; confidence interval 95%, 12.85 and 25.73) (F = 20.951, p < 0.0001). The anti-HBs response was not influenced by coexistent anti-HBc, HCV antibody or HIV antibody seropositivity.