We represent here a 30-year-old woman with cardiac leiomyosarcoma who developed thrombocytopenia and deep vein thrombosis a few days after the start of low molecular weight heparin. Thrombocytopenia improved after replacement of low molecular weight heparin with hirudine. We speculated that cardiac leiomyosarcoma is a predisposing factor for development of heparin induced thrombocytopenia.