The aim of the study was to evaluate the costs and benefits of introducing mass vaccination against hepatitis B in Italy, given the announcement of compulsory vaccination of all newborn babies from 1991. Benefits were calculated by summarizing the clinical course of hepatitis B in Italy and projecting its incidence rates to the next 30 years using Brown's exponential smoothing technique. Incidence rates were then applied to the survivors of a cohort of newborns in 1992, and the total number of cases avoidable through vaccination was derived. Direct and indirect marginal costs were estimated for these cases. The marginal costs of the vaccination campaign were estimated. Cost and benefits were compared for 99 years following the introduction of the vaccination, using an 8 per cent discount rate and a sensitivity analysis. Despite difficulties in data gathering, susceptibility to discounting and estimating costs, results show an unfavourable cost-benefit ratio which is influenced by a declining incidence of the disease. Vaccination of high-risk groups is probably more efficient.