Effect of warfarin prophylaxis on venous thrombosis in patients with PNH granulocyte clone sizes larger than 50% (patients presenting with thrombosis excluded). The 10-year cumulative incidence rate of venous thrombosis in patients with PNH granulocyte clones larger than 50%, not presenting with thrombosis, and not taking warfarin is 36.5%. In comparison, the current thrombosis rate is 0% in patients taking primary prophylaxis (P = .01). Of the 39 patients on primary prophylaxis, 32 had granulocyte clone sizes larger than 50% and could therefore be included in this analysis. A further 2 of these patients were excluded because, having stopped warfarin (1 through personal choice and 1 because of warfarin-associated hemorrhage), they went on to suffer venous thrombosis. Time 0 was the time of presentation with PNH. P value calculated with use of the log-rank test.