Resistance to activated protein C (APCR), in the majority of cases due to the point mutation Arg 506 Gln of the factor V gene, has emerged as the most important hereditary cause of venous thromboembolism. Using an activated thromboplastin time (aPTT) based method in the presence of APC together with a DNA technique based on the polymerase chain reaction, we investigated 37 children with venous (V: n=19) or arterial (A: n=18) thromboembolism and 196 age-matched healthy controls for the presence of this mutation. In the control group 10 children were detected to be heterozygous for the factor V Leiden mutation, indicating a prevalence of 5.1%. 10/19 children (52%) with venous thrombosis and 7/18 (38%) patients with arterial thromboembolism showed the common factor V gene mutation. Additional inherited coagulation disorders were found in 1/10 (V:10%) and 2/7 (A:28%) APC-resistant patients. Inherited coagulation disorders without APCR were diagnosed in 3/9 (V: 33%) and 2/11 (A:18%) children. Furthermore, we diagnosed exogenous risk factors in 6/10 (V: 60%) and 2/7 (A: 28%) children with thrombosis and APCR. These data are evidence that APCR combined with exogenous reasons may play an important role in the early manifestation of thromboembolism during infancy and childhood.