- sample size: in our previous work, the prevalence of previous VTE in first-degree relative of young women with unprovoked (no surgery, no trauma, no immobilization and no cancer) VTE that occurred before 50 years was 9.5% as compared the 5.5% observed in first-degree relatives of young women who had VTE before 50 years with one of these provoking risk factors. For an alpha risk of 5% and a beta risk of 80%, 1000 cases of 200 propositi with VTE and 1000 controls of 2000 propositi without VTE are required; taking in account the proportion of 10% of first-degree relatives that should be classified as "uncertain for VTE", 2200 first degree relatives of 440 propositi are required.