OBJECTIVE To assess the strength of thrombin formation and determine the effects of unfractionated heparin (UFH) in children during cardiac catheterization. BACKGROUND UFH reduces the thrombotic risk related to catheterization but the effects of UFH on the coagulation system in children, and proper monitoring of UFH remain unclear. METHODS We studied 42 patients aged 3-12 years undergoing catheterization. Twenty-seven received UFH (group A) and 15 patients did not (group B). Anticoagulation was assessed by measurements of plasma prothrombin fragment F1 + 2, thrombin-antithrombin (TAT) complexes, D-dimer, activated partial thromboplastin time (APTT), anti-FXa, and prothrombinase-induced clotting time (PiCT). RESULTS Markers of thrombin generation remained low during catheterization in group A. In group B, both F1 + 2 and TAT had increased significantly (P < 0.05) by the end of the procedure versus baseline and versus respective levels in group A. In group A, 15 min after heparinization, APTT was over 180 sec (in all patients), anti-FXa 1.4 U/ml (1.1-2.4 U/ml) and PiCT 1.5 U/ml (1.3-2.4 U/ml). Anti-FXa and PiCT were correlated (R = 0.84, P < 0.0001). CONCLUSIONS Thrombin generation was enhanced in patients who did not receive UFH, which may increase the risk of thrombotic complications. In group A, routine heparinization seemed excessive by all monitoring methods. UFH prevented an increase in prothrombin to thrombin conversion, resulting in unaltered fibrin formation. The current UFH protocol seemed to have no effect on postprocedural activation of coagulation. Further studies are needed to clarify adequate heparin dosing for children during cardiac catheterization to prevent thrombotic complications without predisposing the patient to bleeding complications.