Prothrombin time (PT), activated partial thromboplastin time (APTT) and plasma procoagulant activities were studied in 38 children with nephrotic syndrome in the presence or absence of prednisolone therapy. PT was normal but APTT was prolonged during relapse in untreated patients. Increased factors V, VII, VIII, XI and XIII in both treated and untreated and factor IX in treated patients, as well as decreased factors X and XII in untreated patients, were observed during relapse. These coagulation factor changes were unrelated either to the dose of prednisolone or underlying renal histology and normalized with clinical remission. However, plasma levels of factors II, V, VIII, IX, X and XI were still increased in treated patients. The data suggest that corticosteroids shorten APTT, raise both intrinsic and extrinsic factors, and therefore have favorable and unfavorable effects on the coagulation system in children with nephrotic syndrome.