Children with venous thromboses have greater than 50% likelihood of carrying a genetic thrombophilic defect, and two-thirds of such defects will be a mutation in the factor V gene referred to as Factor V Leiden. Poorly controlled type 1 diabetes mellitus (T1DM) increases the risk for thrombosis. We report a massive arterial thrombosis resulting in unilateral below-the-knee amputation in a 12-yr-old white girl with a heterozygous Factor V Leiden mutation and a 2-yr history of poorly controlled T1DM. This report emphasizes the need to test for thrombophilic defects in children with thrombosis or with a family history of thrombosis. Good metabolic control is especially important in children with T1DM and known thrombophilic defects.