OBJECTIVES: A combination vaccine against hepatitis A and B provides the opportunity for simultaneous protection against both diseases with a single vaccine. This clinical study investigated the reactogenicity and immunogenicity of a combined hepatitis A and B vaccine (Twinrix, GlaxoSmithKline Biologicals, Rixensart, Belgium) in healthy Chilean adults between 18 and 40 years of age. METHODS: In total, 345 healthy, seronegative health care workers were enrolled and randomized to three groups who received one of three lots of Twinrix on a 0-, 1- and 6-month schedules. Blood samples were screened 1 month after each dose for anti-HAV and anti-HBs antibodies. Reactogenicity after each dose was assessed using diary cards. RESULTS: The nature and incidence of symptoms were similar to those reported for other Twinrix studies. Very few symptoms were scored as severe. Upon completion of the vaccination, all subjects had anti-HAV antibodies with titers $6000 mIU/mL, and all but one were protected against hepatitis B, with titers $4000 mIU/mL. CONCLUSIONS: We have demonstrated the high immunogenicity and tolerance of the combined hepatitis A and B vaccine. Combined vaccination has the advantage of offering dual protection with a reduction in the number of injections needed, lower associated costs, and a positive impact on compliance.