A series of 24 bizarre (symplastic, pleomorphic) leiomyomas was analyzed to determine their spectrum of gross and microscopic features and to establish their clinical behavior by obtaining long-term follow-up. Patients' ages ranged from 25 to 51 years (mean 40.7). Maximum dimension of the tumors was about 1-14 cm (mean 4.2), with 83% measuring <5.5 cm and 8% > or = 10 cm. Grossly, the tumors generally resembled ordinary leiomyomas; one third had yellow or tan areas, two had hemorrhages, and a few had focal softening, cavitation or myxoid change. When evaluable (50%), the tumor border was generally circumscribed. Cellularity was 1+ in 21%, 2+ in 58%, and 3+ in 21%. Two tumors were predominantly of epithelioid cell type. Bizarre giant cells were unifocal in 12.5%, multifocal in 37.5%, and diffusely distributed in 50%. More than one third of the tumor contained giant cells in 67%. Degenerative changes occurred in 38%, usually hydropic change. One had infarct-type necrosis. None had tumor cell necrosis. Fibrinoid change of blood vessels occurred in 21%, usually accompanied by chronic inflammation. Mitosis counts ranged from zero to 2.8 mitotic figures (MFs)/10 high-power fields (HPFs) (mean 0.8) by the average count method. By the highest count method, the range was 0-7 MFs/10 HPFs (mean 1.6), with 13% having none and 29% having > or = 2 MFs/10 HPFs. Some pyknotic and karyorrhectic nuclei resembled abnormal MFs. Treatment was hysterectomy in 20 and myomectomy in six (including two with subsequent hysterectomy). Complete follow-up was available in 100%, ranging from 1 to 18.9 years (mean 11.2). Postoperative intervals were > or = 10 years in 58% and > or = 5 years in 83% of cases. All patients were alive and well. Although three patients subsequently developed unrelated second primary malignant neoplasms, none developed recurrence or metastasis of bizarre leiomyoma. Bizarre leiomyomas have a wider range of morphologic changes and mitotic activity than previously documented. This study firmly establishes the benign behavior of bizarre leiomyomas, even for those tumors with high cellularity, numerous widely distributed bizarre cells, and mitosis counts in the 2-7 MFs/10 HPFs range by the highest count method.