An open, randomised study was undertaken to demonstrate the equivalence in immunogenicity and to determine the reactogenicity and safety of two dosing schedules (0, 6 or 0, 12 month) of an adult formulation of a combined hepatitis A and B vaccine containing 720 EL.U. of inactivated hepatitis A antigen and 20 microg of hepatitis B surface antigen (Twinrix, SmithKline Beecham Biologicals, Belgium) in 240 healthy volunteers aged 12-15 years. The vaccine was well tolerated when administered using either vaccination schedule. At month 7, 98.1% of subjects completing the 0, 6 month vaccination schedule were seroprotected against hepatitis B (anti-hepatitis B surface antigen (anti-HBs) > or =10 mIU/ml) and 100% were seropositive for anti-hepatitis A virus (anti-HAV) antibodies (i.e., > or =33 mIU/ml). The corresponding geometric mean titres (GMTs) were 2791 mIU/ml for anti-HBs and 5992 mIU/ml for anti-HAV antibodies. At month 13, 97% of subjects assigned to the 0, 12 month vaccination schedule were protected against hepatitis B and 99% were seropositive for anti-HAV antibodies. The corresponding GMTs were 4340 and 8472 mIU/ml, respectively. A combined response (i.e., subjects, who were seropositive for anti-HAV antibodies and seroprotected for anti-HBs antibodies) was achieved in 98% of subjects vaccinated according to the 0, 6 month interval and in 96% of subjects vaccinated using the 0, 12 month schedule. The reactogenicity of both vaccination schedules was also equivalent. The results thus show that the combined hepatitis A and B vaccine can be administered using flexible vaccination intervals, which make it suitable for use in large-scale hepatitis immunisation programmes.