Objective. The objective of our study was to evaluate the age, sex, clinical conditions, family history, site, catheter association, means of radiologic evaluation, development of pulmonary involvement, prevalence of antithrombin III, protein C and protein S deficiencies, and lupus anticoagulants in children who suffered a thrombotic event. Methods. Data were collected on children over 1 month of age who had or developed a thrombotic event from 1987 through 1993 at two pediatric centers. Results. Sixty-one children (mean age, 10 years) suffered a thrombotic event. Males and females were equally affected. A variety of clinical prothrombotic conditions similar to those described in adults could be identified for two thirds of the children. Family history was positive in seven children. The primary thrombotic site for two thirds of the children was the central nervous system and other centrally located blood vessels. Diagnosis of the primary thrombotic site was primarily by ultrasound. A central vascular access device was associated with 25% of thromboses. Lung involvement occurred in 20%. Two thirds of the children were evaluated for a lupus anticoagulant and a deficiency of protein C and protein S; two thirds had one of these diagnosed. For further analyses, children without an underlying prothrombotic systemic illness or precipitant at the time of thrombosis (n = 20) were compared to those with these conditions (n = 41). Central nervous system thromboses were significantly increased in the children without prothrombotic conditions. The prevalence of a deficiency of protein C or protein S or the presence of a lupus anticoagulant approached 90% in the group without prothrombotic conditions as compared with 50% in the other group. Conclusion. We conclude that prospective multicenter pediatric thrombosis studies are warranted to confirm our preliminary findings of a high incidence of lupus anticoagulants and protein C and protein S deficiency in children with thromboses.