Melanocytomas are pigmented tumors of the uvea and optic nerve head composed of plump polyhedral melanocytes which have been regarded as nevus cells. There are few reports of malignant change in these usually benign lesions. This study documents 16 cases of malignant melanoma of the uvea, eight of which were characterized by the presence of plump polyhedral melanocytes in and around the tumor. The melanocytoma cells were aggregated throughout the tumors and/or at the margins of the lesions; these cells did not appear between the melanoma and the sclera, as has been reported with spindle-type nevus cells. Electron micrographs of these plump melanocytes had the characteristics of common uveal melanocytes, while the tumor cells had features of malignant melanoma cells. The clinician should recognize the potential of melanocytomas for undergoing malignant change and should follow patients who have this lesion with this in mind. Article is followed by comments of Dr. L. E. Zimmerman.