We describe the clinical presentation and pathological features of a cardiac sarcoma that occurred within the right atrial cavity of a 14-year-old boy. The patient presented with a 2-month history of increasingly severe heart failure. Tumor tissue obtained by biopsy showed light microscopic features of leiomyosarcoma. Immunohistochemical reactions and ultrastructural studies provided evidence of tumor cell differentiation along three cell lines--smooth muscle, fibroblastic, and endothelial--suggestive of origin from a multipotent subendothelial reserve cell. Postmortem examination disclosed a large necrotic tumor filling the right atrial chamber. Implants were present on the pericardium, but there were no other metastatic lesions. The difficulty of making a diagnosis of these rare neoplasms when the tumor is small and potentially curable results in a grim prognosis.