A total of 360 pediatric patients aged 1 month to 18 years were diagnosed as having thrombosis between January 1998 and April 2003. Of these patients, those aged 11–18 years (n = 131) were regarded as adolescents and the rest as children. The proportion of adolescents in the whole group excluding the neonates was 36%. The peak age of diagnosis in adolescents was 11–14 years, comprising 58% of all thrombotic events in adolescents. In 73% of the adolescents, there was at least one risk factor. The four most common underlying disorders were infection, malignancy, connective tissue and cardiac disorders, in decreasing order of frequency. In children, on the other hand, infection was followed by congenital heart disease, malignancy and liver disease. Three common types of thrombosis in adolescents were deep venous thrombosis, cerebro-vascular events and portal venous thrombosis, while cerebro-vascular events were the most common in children. The frequency of factor V G1691A mutation in the adolescents (22.1%) was significantly higher than that found in a group of healthy controls (7.4%) and this mutation was associated with a 3.6-fold increase in the risk of developing thrombosis (95% confidence interval, 1.4–9.0). The carrier frequency of prothrombin G20210A mutation (3.1%) in adolescents did not differ significantly from that of the healthy population (2.3%) and no association was observed between this mutation and a risk of developing thrombosis in this group (odds ratio, 1.3; 95% confidence interval, 0.2–7.5). The rate of recurrent thrombosis was 6%.