A hypercoagulable state and the risk of thromboembolism in both arterial and venous circulation is a relatively frequent and serious feature of nephrotic syndrome (NS) in children and adults. The aim of this study was to evaluate the coagulation states of children with NS before and after corticosteroid (CS) therapy and to compare the results with a healthy control group. The first group consisted of 49 nephrotic children (30 boys and 19 girls) with a mean age of 6. 5+/-4.9 years (range 1-16 years). The control group included 17 healthy children (9 boys and 8 girls). At the time of admission, all patients were evaluated for the presence of clinical thromboembolism, hematological and biochemical indicators of a hypercoagulative state, and renal disease. This was repeated after CS treatment. Deep vein thrombosis was observed in 2 nephrotic patients who had very low plasma antithrombin III (AT III) levels and fibrinogen levels above 750 mg/dl. Thus, the prevalence of thromboembolism was 4% in our pediatric nephrotic population. The mean AT III level of the study group was 68.2+/-23.4% at the onset of the disease, which was significantly lower than the level of the control group (84.0+/-7. 6%). Plasma AT III levels increased to 74.4+/-15.3% after CS treatment, which correlated with the serum albumin levels. However, there was no correlation with urinary protein excretion. Protein C levels were higher than controls during all stages of the disease in both steroid-responsive and -unresponsive patients. The mean protein S level was similar in both groups. Plasma fibrinogen and cholesterol levels were significantly higher in the study group but decreased to within normal limits with remission. Our study suggests that thromboembolic complications are not infrequent in children with NS, and may be related to low plasma AT III and albumin and high fibrinogen and cholesterol levels.