Between 1963 and 1987, 131 patients with benign gastrointestinal stromal tumors, primarily leiomyomas, were treated at the Massachusetts General Hospital. Eighty per cent of tumors were located in stomach or small intestine. Two thirds of the tumors were discovered in symptomatic patients before operation by a variety of diagnostic studies. A mitotic index (MI) was determined for each tumor, defined as the number of mitoses per 50 high-power microscopic fields. Only gastric and small intestine tumors had MIs more than 2. Tumors were treated by conservative excision in 67% and radical excision in 33%. At a median follow-up of 6 years there were no local recurrences. No patient with a tumor discovered incidentally has recurred. Three symptomatic patients have died of metastatic liver disease. Each patient with recurrence had a tumor with MI of 2 or more, which represents a recurrence rate of 16% in this group. We conclude that symptomatic gastric and small intestine tumors having two or more mitoses per 50 high-power fields carry a significant risk for recurrence, and that routine pathologic assessment of MI may identify a subset of patients who would potentially benefit from close follow-up and consideration for further therapy.