Between 1957 and March 1991, 106 patients with 110 neoplasms that originated in the heart were treated surgically at the Mayo Clinic and had pathologic material available for review. The study group consisted of 39 male and 67 female patients, who ranged in age from 2 to 80 years. Benign atrial myxomas (64 in the left atrium and 16 in the right atrium) were the most commonly encountered neoplasm. The other benign tumors were nine fibromas, five lipomatous tumors, seven valvular fibroelastic papillomas, and one cardiac hamartoma (so-called oncocytic cardiomyopathy). In addition, eight patients had a primary cardiac malignant lesion: angiosarcoma, leiomyosarcoma, and malignant fibrous histiocytoma in two patients each and sarcoma (not otherwise specified) and osteogenic sarcoma in one patient each. The angiosarcomas originated in the right atrium, and the other malignant tumors originated in the left atrium. The histologic feature that most frequently predicted an adverse clinical outcome was the presence of mitotic figures, although highly cellular tumors and those with necrosis also tended to have a malignant course.