After observing a child with systemic onset juvenile rheumatoid arthritis (S‐JRA) who developed purpura fulminans in association with disseminated intravascular coagulation, with subsequent gangrene and autoamputation, we undertook a prospective study of coagulation parameters in children with JRA. Ten consecutive children with S‐JRA, 10 children with rheumatoid factor‐negative, polyarticular juvenile rheumatoid arthritis (P‐JRA), and 10 age‐ and sex‐matched controls were studied. Routine coagulation screening tests were performed, as were tests for plasma fibrinopeptide A (a sensitive measure of intravascular thrombin generation), factor VIII‐related antigen (an endothelial cell protein), and platelet factor 4 (a plateletsecreted protein). Our studies suggest that activation of intravascular coagulation is common in systemic onset JRA, but not in rheumatoid factor‐negative, polyarticular disease. The coagulopathy may cause severe morbidity. In addition, marked elevations of plasma factor VIII‐related antigen suggest perturbation of endothelial cells and vascular involvement in S‐JRA, but not in P‐JRA. Normal ranges of platelet factor 4 indicate that intravascular platelet consumption does not occur in either type of JRA, despite the thrombocytosis common in both.