A severe heparin-induced thrombocytopenia is reported in a child suffering from acute myocarditis. Thrombocytopenia (80.10(9)/l) occurred on day 5 (D5) of heparin therapy which was thus discontinued during 8 hours but reintroduced on the view of a negative platelet aggregation test using standard heparin. On D10, while thrombocytopenia persisted the presence of two intra-cardiac thrombi led to replace standard heparin by low molecular weight heparin (Fraxiparin), associated with vitamin K antagonist (phenindione). On D12, a 3rd intra-cardiac thrombus required immediate discontinuation of Fraxiparin therapy. Platelet aggregation tests performed on D14 were positive in the presence of standard heparin and of low molecular weight heparin, thus demonstrating the existence of thrombocytopenia induced by standard heparin and secondly by low molecular weight heparin. This observation led the authors to discuss the diagnosis and the therapeutic management of heparin-induced thrombocytopenia.