Malignant blue nevus.

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.