A 16 year-old patient presented with acute pain in his right thigh. The thigh was inflamed and swollen (10 cm difference in diameter from the left thigh). Ultrasonography (Doppler bimode) showed occlusion of the deep femoral vein extending to the level of the groin. Local thrombolytic treatment with urokinase (Ukidan) followed by μMBH in therapeutic dosage led to recanalization of the venous network. Complete investigation of the young man’s hemostatic mechanism revealed that he is a heterozygote for the mutation of the V-Leiden product.