A combined hepatitis A and B vaccine, Twinrix, in a paediatric formulation for ages 1-15 years and in an adult formulation for those ages 16 years and older, became commercially available in Turkey as well as in many countries. It is administered according to a three-dose schedule (0, 1 and 6 months). A reduction in the number of doses would improve the compliance rate and reduce administration costs. Therefore, we planned a trial evaluation of the immunogenicity, safety and reactogenicity profile of a high-dose combined hepatitis A and B vaccine, administered in two doses, compared with the profile of a paediatric-dose combined vaccine, administered in three doses, in healthy children aged 6-15 years. One hundred children were randomly attributed to the two study groups. The first group (paediatric-dose vaccine group) received the licensed Twinrix Paediatric, at months 0, 1 and 6; the second group (high-dose vaccine group) received the high-dose vaccine, following a 0, 6 months schedule. The reactogenicity was assessed after each vaccine dose. The immunogenicity was evaluated by testing for anti-HBs and anti-HAV antibodies. Seroconversion rates and geometric mean titres (GMTs) were compared. Both formulations of the combined vaccine were well tolerated. The high-dose combined vaccine administered in two doses, elicits satisfactory immunogenicity profiles, similar to those elicited by the paediatric vaccine administered in three doses. On completion of the vaccination schedule in the two groups all children were protected against hepatitis B and immune for hepatitis A. Anti-HAV GMTs after completion of the vaccination schedule were 7163 ml U/ml in the paediatric-dose group, 8241 ml U/ml in the high-dose group; anti-HBs GMTs were 8679 and 4583 ml U/ml, respectively. These results indicate that a two-dose schedule, compared with the standard three-dose schedule, offers fewer injections for satisfactory protection against the two infections. This means fewer clinic visits, lower administration costs, better compliance, and higher coverage rate. Therefore, this two-dose schedule can be considered an appropriate regimen for the immunization of children and adolescents against hepatitis A and B infection, in the context of school-based immunization programmes. (c) 2004 Elsevier Ltd. All rights reserved.