In both children and neonates with venous thrombosis, one or more predisposing risk factors can generally be identified. Underlying prothrombotic medical conditions are common in these patients. In addition, a "trigger factor" such as a catheter, surgery or trauma is usually present. However genetic and acquired coagulation abnormalities are also identified in children with venous thrombosis who are appropriately studied. A careful family history and assays for the LA, AT-III, protein C, protein S and the factor V Leiden mutation should be part of the evaluation of infants and children with venous thrombosis.