Smooth muscle tumors of the scrotum are very uncommon, and those with degenerative-appearing atypia, variably designated as "atypical," "symplastic," or "bizarre" leiomyomas, are extremely rare with only 11 cases in the literature. Given their rarity, the diagnostic criteria and prognosis of symplastic leiomyomas are not well established. We describe 9 cases of scrotal symplastic leiomyomas and compare their histopathologic characteristics to 10 usual leiomyomas and 5 leiomyosarcomas of the scrotum. The preoperative diagnosis was scrotal tumor or cyst in all cases. The mean age was 46 years (range, 32 to 60 y) for usual, 59 (range, 48 to 79) for symplastic leiomyomas, and 57 (range, 49 to 65) for leiomyosarcoma. Submitting diagnoses for symplastic leiomyomas were: atypical spindle cell lesion (n=3); probably leiomyosarcoma (n=1); leiomyosarcoma (n=1); and none given (n=4). Symplastic leiomyomas were diagnosed when there was moderate-severe cytologic atypia, yet was degenerative-appearing with multinucleation or smudged chromatin in the setting of low nuclear/cytoplasmic ratio, low cellularity, and no mitotic activity. The mean size was 1.0 cm for usual leiomyomas, 1.0 cm for symplastic leiomyomas, and 2.0 cm for leiomyosarcomas. Leiomyosarcomas had high nuclear/cytoplasmic ratio, high cellularity, nuclear pleomorphism, and hyperchromasia. Five of 10 usual and 3/9 symplastic leiomyomas showed at least 1 ill-defined border simulating infiltrative growth. Three leiomyosarcomas were grade 1, and 2/5 were grade 2. Resection margins were positive in 5/10 usual and 3/9 symplastic leiomyomas and in 1/5 leiomyosarcoma. Ki67 labeling in usual leiomyomas was on average 2.4% (range, 1% to 5%) and in symplastic leiomyomas was 1.8% (range, 1% to 5%). Mitoses were absent in all cases of usual and symplastic leiomyomas. Mitotic figures averaged 4.7 (range, 1 to 7) and 13.5 (range, 7 to 20) per 10 HPF for the grade 1 and 2 leiomyosarcomas, respectively. None of the symplastic leiomyomas recurred after a median follow-up of 27 months. The 2 patients with grade 2 leiomyosarcoma had no evidence of metastases at 6 and 7 months follow-up, respectively. Scrotal symplastic leiomyomas may have an ill-defined infiltrative border, which along with their atypia mimic malignancy. Ki67 is low in symplastic leiomyomas, which along with their favorable follow-up and experience in other organs justifies a benign diagnosis. High cellularity and high mitotic activity are the most reliable features for the diagnosis of scrotal leiomyosarcoma.