OBJECTIVES: Most international pharmacoeconomic guidelines advise not to include future unrelated medical costs in economic evaluations. Recently, arguments in favor of inclusion of such costs are gaining support in health economic literature. However, correct estimations of these costs do not yet exist. This paper describes the methodology with which we developed a toolkit to adjust incremental cost-effectiveness ratios (ICER) for future unrelated medical costs, and presents an application of the toolkit by adjusting the ICER of universal hepatitis B vaccination of newborns. METHODS: We defined three variables that predict individual health care expenditure best: age, sex and time to death. However, the relation between health care expenditure and these three variables depends on the lethality of diseases and on health care setting. For the development of the toolkit, different cross-sectional and longitudinal databases were used. First, the 2003 Dutch Cost of Illness study has been employed. Furthermore, insurance claims data and longitudinal record linkage data were used. This allowed us to estimate how the relation between time to death and health care costs is altered if costs of related diseases are excluded. To apply the toolkit, survivor curves for the intervention and control groups are needed. For hepatitis B vaccination of newborns, future unrelated medical costs (i.e. all costs not related to hepatitis B infection and sequelae) are added using the differences between survivor curves of Hepatitis B infected persons and population survivor curves. RESULTS: Without the inclusion of future unrelated medical costs, Hepatitis B vaccination costs € 900 per life year gained. Adjusting the ICER for these costs results in an increase to approximately € 5.700 per life year gained. CONCLUSIONS: We conclude that the adjusted ICER provides more evidence-based and transparant information about the real cost-effectiveness of an intervention.