A 47-year-old woman who had had a flat, hyperpigmented lesion of the optic disc and juxtapapillary choroid for at least 17 years experienced acute visual loss. This was caused by rapid growth of a pigmented tumour emanating from the original lesion. Histopathological examination of the enucleated globe revealed a large malignant melanoma originating from a melanocytoma (magnocellular nevus) within the optic nerve and peripapillary choroid. This case represents a very rare instance of malignant transformation of a normally benign tumour.