Malignant blue nevus is uncommon compared to its benign counterpart and is regarded as a rare form of malignant melanoma. We report the clinical and histological findings in eight cases. Histologically, all eight specimens showed no epidermal involvement and had contained within or were adjacent to portions of blue nevus or cellular blue nevus. Proliferation of bundles of bipolar spindle shaped cells with marked cellular atypia, mitotic figures, foci of necrosis, and inflammatory cell infiltrate were noted. Two of the cases were studied by DNA flow cytometry and the populations of tumor cells were found to be diploid. Two cases have died secondary to metastasis. Although malignant blue nevi may not behave as aggressively as nodular malignant melanoma, they have definite potential to do so and therefore should be removed by wide surgical excision.