OBJECTIVE To evaluate the immunogenicity and reactogenicity of two lots of a combined hepatitis A-hepatitis B vaccine (HAV, HBV) in healthy 15 to 18 year olds. METHODS This was a double-blind, randomized clinical study. Vaccine was administered into the deltoid at 0, 1, and 6 months. Immunogenicity was assessed by anti-HAV and anti-HBs antibody levels at 2, 6, and 7 months after the first vaccine dose. Reactogenicity was assessed through use of 3-day diary cards following each vaccination, plus recording other unsolicited reactions. RESULTS A total of 160 adolescents were vaccinated; 155 who were seronegative for hepatitis A and B at baseline and who completed the study were included in the immunogenicity analysis. The vaccine was well tolerated; most side effects were local, of low intensity and short duration. Good immunogenicity was determined by antibody titers. High rates of seropositivity (99.4%) were achieved after two doses against HAV, and after three doses for anti-HBs (seroprotection = 98.7%). CONCLUSIONS This combination vaccine will be useful for immunizing selected high-risk groups in developed countries. In countries where endemicity is low for both diseases, targeting students prior to risk of acquisition would be a feasible preventive strategy.