Deep venous thromboses following sclerotherapy are very rare, yet occur more frequently in thrombophilic patients. We have here described two cases in which post-sclerotherapy deep venous thrombosis revealed a mutation in the prothrombin gene (Factor II). We have further explained the principles of sclerotherapy with lauromacrogol (polidocanol). We have finally summarized the causes of thrombophilia and provided practical recommendations regarding sclerotherapy in thrombophilic patients.