Longitudinal melanonychia displaying features of keratinized acanthoma is described in two patients. In both cases, a pigmented band consisting of a subungual keratinized epithelial ridge originated in the nail bed. We have been able to show, using the clinical and histological findings, that the origin of the pigment is linked to its synthesis within the acanthoma of the nail bed. These lesions are reminiscent of pigmented seborrhoeic keratosis. The horny cysts are replaced, because of the special physiological longitudinal arrangement of the ridges in the nail bed, by a single, prominent, longitudinal, keratinized and pigmented mass.