The role of the family history as a tool for the diagnosis of inherited thrombophilia has not been established. Several authors have indicated that a positive family history is not a good predictor of inherited abnormalities such as antithrombin III deficiency, or deficiencies of protein C or protein S. We have tried to approach the family history in a quantitative way. To this end we used the cumulative incidence data of thrombosis in the general population and also in a population of protein C deficient families to estimate the expected number of symptomatic subjects in a family under both the hypothesis of inherited thrombophilia and the null-hypothesis. Although a number of assumptions underlying our calculations need to be verified and probably adjusted before any truly quantitative meaning can be assigned to this approach, we feel that the family history is a useful diagnostic test for inherited thrombophilia if it is used in a critical way.