To elucidate the histologic features and biologic behavior of malignant blue nevus (MBN), 12 cases were analyzed in which the tumor showed no junctional activity and arose in a blue nevus background. Seven patients were men and five were women; their mean age was 48.8 years. Eight lesions were on the scalp, and no patient had a family history of melanoma. The histologic appearance of most lesions was a nodule or nodules of malignant cells in a blue nevus. Mitoses were present in all lesions with atypical forms in eight; however the mitotic rate exceeded two per ten high-power fields in only one lesion. Four lesions had necrosis, and four had a heavily pigmented malignant component. Four patients had recurrences; ten patients had metastases, and eight died of their disease. Therefore MBN is an aggressive neoplasm.