Objective:To determine whether tumors meeting the criteria of Hendrickson and Kempson for uterine smooth‐muscle tumors of uncertain malignant potential have a natural history different from those of leiomyomas and leiomyosarcomas.Methods:Tumors with five to ten mitoses per ten high‐power fields and with mild or moderate cellular atypia were classified as tumors of uncertain malignant potential. Tumors with two to four mitoses per ten high‐power fields and severe cellular atypia would also be classified as tumors of uncertain malignant potential, but we had no tumors that fell into this latter group. Forty‐seven women with leiomyosarcoma or smooth‐muscle tumors of uncertain malignant potential were identified. Paraffin‐embedded blocks were recut, and hematoxylin and eosin‐stained sections were studied for mitotic counts and cellular atypia. Statistical analysis usedX2, Fisher exact test, Studentttest, and Kaplan‐Meier life table analysis.Results:Fifteen tumors were classified as uncertain malignant potential and 32 as leiomyosarcomas. The patients with leiomyosarcoma were significantly older and more likely to present with extrauterine disease. Those with tumors of uncertain malignant potential had a 5‐year disease‐free survival of 66% and overall survival of 92%, compared to 28 and 40%, respectively, for leiomyosarcomas; these differences were statistically significant. Patients with tumors of uncertain malignant potential tended to have a protracted clinical course after development of recurrence, and several survived longer than 5 years with metastatic disease.Conclusions:Patients with five to ten mitoses per ten high‐power fields and mild to moderate cellular atypia had a prognosis significantly better than that of patients with leiomyosarcomas. In this group, only 27% developed a recurrence, and after recurrence they tended to have a protracted course. Some of these tumors do have a very aggressive course, and the term “uncertain malignant potential” is appropriate.(Obstet Gynecol 1994;83:1015‐20)