Two patients, whom we had under surveillance since 1969, were found to develop photo-onycholysis from orally administered 8-methoxypsoralen and sunlight irradiation. Morphological examination of the nail bed revealed characteristics of a microtrauma. The photosensitizing effect of the drug induced the generation of numerous binucleate, tetranucleate and multinucleate epithelial cells as well as numerous binucleate and tetranucleate fibroblasts in the dermis.