Sixty-three children with thrombosis were screened for factor V Leiden (FVL) and the mutation was found in 20 patients, 5 of whom were homozygous for FVL. The clinical features of 15 heterozygous and 5 homozygous patients with factor V Leiden mutation were analyzed. Additional risk factors were found in 19 of these 20 patients (95%) with FVL mutation. The most frequent predisposing exogenous factor was infection. The majority of children with FVL mutation had had several associated disorders prior to their vascular occlusion, such as homocystinuria, Behçet's disease, collagen tissue disorders, and hypereosinophilia. Homozygous mutation in two children led to amputation of legs due to purpura fulminans. Cerebral thrombosis as cerebral infarct was frequent among children with FVL mutation.