Twenty-four children with juvenile rheumatoid arthritis (JRA) and 10 children with postinfectious arthropathies were investigated for markers of blood coagulation and fibrinolytic activity: Prothrombin fragment 1+2 (F1+2), thrombin-antithrombin complex (TAT), and D-Dimer were measured using solid phase enzyme linked immunosorbent assays (ELISA). Results were compared with clinical and conventional laboratory signs of disease activity. F1+2, TAT, D-Dimer, and fibrinogen were significantly elevated in children with JRA as compared with healthy children and children with postinfectious arthropathies. F1+2, TAT, and D-Dimer correlated significantly with disease activity, assessed by determination of the joint index score and C-reactive protein (CRP). The study demonstrates a subclinical activation of the haemostatic system in children with JRA correlating with disease activity, which might be caused by the action of several immunomediators on cells (monocytes, endothelial cells) playing a role in the regulation of blood coagulation activity.