Two cases of slowly progressing pigmentation of a fingertip are presented. The lesions were macular and involved the nail without destroying it. They had the clinical and histologic characteristics of lentigo maligna (melanotic freckle) type of precancerous melanosis as contrasted with the superficially spreading (pagetoid) malignant melanoma. Electron microscopic examination showed the features of malignant cells of melanocytic type and permitted differentiation from nevus cells. Melanosomes were found to be elongated and often rod-shaped. Conglomeration of melanosomes, presence of cytoplasmic vacuoles and polysomes, and the characteristics of mitochondria and nuclei were indications of the malignant nature of the cells. The decision for relatively conservative treatment in one case, and pure observation in the other case, was based on identification of the lesions as melanocytic rather than nevocytic precancerosis.