Thrombophilia screening is a time-consuming and expensive procedure. Information about thromboembolism among relatives may be a simple method to identify those at risk of having thrombophilia. In a cross-sectional clinical study the authors have investigated the role of such family histories to detect children with thrombophilia. In 202 children a family history of venous or arterial thromboembolism was recorded. The participants were either children with congenital heart defects at the time of a scheduled cardiac catheterization (n=134) or children with newly diagnosed cancer (n=68). Questions were answered, on the spot, by the parents, at the same time as blood samples for thrombophilia screening were taken. Questions about family history of thromboembolism were completed in 184 children, of whom 114 children (62%) were positive, and 35 of these had relatives with venous thromboembolic events. Only one child had an affected first-degree relative. Thrombophilic alterations were observed in 60 children (30%), and 25 of these were defined as inherited. A positive family history of venous origin increased the relative risk of a child having inherited thrombophilia to 2.35 (95% confidence interval 1.1--5.2). Information about familial arterial thromboembolism was not useful in spotting children with prothrombotic risk factors. These results indicate that questioning about family history of venous thromboembolism may identify children with genetic thrombophilia, but the association is not strong. The authors encourage similar larger-scale studies to enlighten the issue fully.