Ovarian malignant melanoma (MM), primary or metastatic, is an extremely rare tumor and in the absence of a previous diagnosis can represent a diagnostic challenge. We present the clinicopathologic and immunohistochemical features of 23 cases seen in our institution over a period of 40 years (1962-2001). The patients' age ranged from 14 to 53 years (mean 35.7 years). Ethnicity was known in 19 patients: 14 white, 4 Hispanic, and 1 black. A previous history of MM was definitively obtained in 14 patients; in these cases, the interval between the primary MM and the ovarian metastasis ranged from 15 to 228 months (mean 77.7 months). The tumor was unilateral in 19 and bilateral in 4 cases. The tumor size ranged from 4.5 to 23 cm (average 10 cm); the melanoma arising in a cystic teratoma was 0.2 mm in thickness. The tumor was grossly pigmented in 8 cases (35%). The architectural pattern was nodular (8 cases), diffuse (6 cases), nodular and diffuse (5 cases), nested (3 cases), and lentiginous arising in a teratoma (1 case). Follicle-like spaces were seen in 8 cases, pseudo-glandular areas in 1 case, pseudo-myxoid areas in 1 case, and cords in 1 case. The tumor cell type was epithelioid in 19 cases, spindled in 2 cases, mixed epithelioid and spindled in 1 case, and small cell in 1 case. Nucleoli were prominent in 18 cases, and nuclear inclusions were present but rare in the majority of cases. Nuclear grooves were seen in 3 cases. Necrosis was extensive in 8 cases, focal in 10 cases, and was absent in 5 cases. In 8 cases, initial diagnoses included sex cord stromal tumor, germ cell tumor, sarcoma, or undifferentiated carcinoma. S-100 was positive in 18 of 19 cases, HMB-45 in 17 of 20 cases, MART-1 in 13 of 15 cases, tyrosinase in 10 of 15 cases, and Mitf in 8 of 14 cases. Inhibin was positive in 3 of 14 cases. Calretinin was focally positive in 1 of 12 cases. Treatment performed in 18 of the cases are as follows: oophorectomy with/without chemotherapy (10); total abdominal hysterectomy with bilateral salpingo-oophorectomy with/without chemotherapy (6); vaginal hysterectomy, bilateral salpingo-oophorectomy, and chemotherapy (1); and total abdominal hysterectomy with salpingo-oophorectomy (1). Follow-up ranging from 2 to 96 months was available in 18 patients. All but one had metastases in other organs, most often in the lungs. Thirteen patients died of disease (range 2-76 months), 3 are alive with disease (6-18 months), and 2 have no evidence of disease at 24 and 96 months; one was the patient with melanoma arising within a teratoma. In conclusion, MM involving the ovary is a rare disease, predominantly seen in women of reproductive age, and is associated with a poor prognosis. The tumor is most often metastatic from another site and is unilateral in most cases. Nodular or diffuse pattern and epithelioid cell type are most frequently seen, and the tumor can be mistaken for germ cell and sex cord stromal tumors. S-100 is the most sensitive marker. MART-1 was positive in the few cases that were negative with HMB-45. Inhibin can be focally positive in some cases.