Ovarian leiomyomas are uncommon and have not been systematically studied. The clinical and histopathologic features of 15 ovarian leiomyomas were evaluated, including their clinical presentation, size, cellularity, mitotic index, presence of degeneration, and hyalinization. The mean age of women with ovarian leiomyomas was 45.8 years. The presenting sign was an adnexal mass in 7 (47%), uterine leiomyomas in 4 (26%), contralateral adnexal mass in 2 (13%), and other signs in 2 (13%) women. Thirteen leiomyomas had mitotic indices < 1/10 high-power-fields (HPFs), including one cellular leiomyoma, and 2 had mitotic indices between 1 and 2/10 HPFs. The cellular leiomyoma and those with a mitotic index of > or = 1/10 HPFs had an average size of 3.4 cm. Two leiomyomas exhibited central degeneration, and hyaline change was present in 5 leiomyomas, 4 of which were at least 4 cm in size. The majority of ovarian leiomyomas (78%) were associated with uterine leiomyomas; hyperplastic endometrial polyps were present in 2 cases. Follow-up (mean, 9.6 months) was available in 8 cases, and all had no evidence of recurrence. In conclusion, ovarian leiomyomas exhibit a spectrum of features that are similar to uterine leiomyomas. Although the prognosis is presumed to be good, the short follow-up period of this series precluded definitive evaluation of their natural history.