The light and electron microscopic observations of a case of malignant blue nevus arising in the skin of the lumbar region of a 35-year-old man are presented. The patient died of widespread metastatic disease one year after complete surgical excision of the primary tumor and 17 months after the primary lesion was noted. Histologically, the primary tumor and the metastases fulfill the criteria for the diagnosis of malignant cellular blue nevus. Electron microsopic studies of the tumor metastases do not support the few recently reported observations of the ultrastructural resemblance of the tumor cells of malignant blue nevus with Schwann cells. The findings described in this case support the melanocytic nature of the blue nevus cells.