The term "malignant blue nevus" refers to a rare and heterogeneous group of melanomas that arise in several clinical settings. This includes melanomas arising in association with a common or cellular blue nevus and those arising de novo and resembling cellular blue nevi. We reviewed the clinicopathologic features of 10 cases of malignant blue nevi. Six cases proved to be de novo melanoma mimicking cellular blue nevus, but lacking a clear-cut benign component. Two melanomas arose in association with a common blue nevus, and two with a cellular blue nevus. The patients' (5 males, 5 females) ages ranged from 11 to 77 years (average age, 48.1 years). The head and neck was the most common location (6 of 10 patients), with five scalp tumors. Four tumors were located on the trunk; none was located on the extremities. Tumor size ranged from 0.5 to 2.2 cm (average size, 1.1cm). Most lesions had been present for many years before surgical removal. Pigmented dendritic cells were observed in 9 of 10 cases. The malignant and benign components were easily distinguished in the four cases that arose in association with a common or cellular blue nevus. Abrupt transition between a benign blue nevus and melanoma was readily recognized at scanning magnification as distinctive nodules of epithelioid to spindled cells with a sheet-like growth pattern. In all cases, malignancy was evidenced by increased mitotic rate, necrosis, nuclear atypia, pleomorphism, hyperchromasia, and prominent nucleoli. All 7 patients with follow-up information experienced recurrence (3 patients) or metastasis (4 patients). Three patients died of disease. Malignant blue nevus is a heterogeneous group of melanomas that are highly aggressive and often lethal, with a propensity for metastasis to the lymph nodes and lungs.