Primary cardiac tumors are very uncommon. Seventy five percent of them are benign tumors and these are mostly myxomas. The malignant cardiac tumors, the majority of which are undifferentiated sarcomas, comprise up to 25% of all cardiac tumors. A primary malignant sarcoma of the myocardium is exceedingly rare. Thus, there have been very few such cases reported in the literature. We present here a case of a 15 year old man who had complaints of orthopnea and increasing exercise intolerance over a one month period. Transthoracic echocardiography demonstrated a well demarcated huge mass with left ventricular inflow obstruction on the posterior wall of the left atrium. The patient's symptoms were relieved by surgery. The histological diagnosis was an unclassified spindle cell sarcoma.