Assessment of the biological potential of smooth muscle tumors can be difficult and depends primarily on tumor site, stage, and histologic parameters. In this study, we examined the clinicopathologic and immunohistochemical features of 55 noncutaneous inguinal smooth muscle tumors of women (age range, 20 to 82 y; median, 57 y). Histologically, 23 tumors were considered as leiomyomas. They showed low mitotic activity (range, 0 to 6 mitoses/10 high-power fields, without atypical mitotic figures), minimal cytologic atypia, and absence of coagulative necrosis. Fifteen of these tumors histologically resembled conventional uterine leiomyomas and 8 resembled their variants: lipoleiomyomas (n = 2) and epithelioid variants (n = 6). The mean size was 7.8 cm, and half of the tumors with specified location arose in association with the round ligament. Immunohistochemical expression of estrogen receptor (ER) and/or Wilms tumor protein (WT1) was detected in most cases (83%), supporting Müllerian derivation. Follow-up data (range, 10 to 29 y; median, 13 y) on 11 patients showed that all were alive without disease or death from unrelated causes. The second group, classified as leiomyosarcomas, consisted of 32 mitotically active smooth muscle tumors, almost invariably with atypical mitotic figures, and exhibiting significant cytologic atypia. These patients were older than those with leiomyomas, and their tumors were mostly subcutaneous with a mean tumor size of 5.4 cm. Two leiomyosarcomas showed a femoral vein origin, but none were associated with the round ligament. All but 3 leiomyosarcomas were negative for ER. Follow-up data on 13 patients (range, 2 mo to 30 y; median, 4.5 y) showed that 5 died of metastatic sarcoma. Six individuals were alive without disease (median, 16 y), and 2 died of unrelated causes. In conclusion, inguinal smooth muscle tumors in women are a dichotomous group. They consist of ER/WT1-positive Müllerian-type leiomyomas resembling uterine leiomyomas with an excellent prognosis and conventional LMSs that are usually ER/WT1-negative and show a variable malignant course. Separation of these 2 categories is important for prognostication and optimal patient management, and is aided by immunohistochemical studies for ER and WT1.