We have studied 15 patients with photo-onycholysis induced by tetracyclines, psoralens, or fluoroquinolones. Three distinct clinical subtypes of onycholysis were seen. Type I showed a half-moon-shaped separation that was concave distally. Type II had a circular notch opened distally and shaped as if the distal nail plate had acted as a convex lens. In type III the changes were located in the central part of the nail bed with no connection to the margins. Ultraviolet (UV) irradiation of normal fingernails with various wavelengths showed that 3% to 20% of the irradiation could penetrate the nail. The different patterns of photodamage might be caused by the nail acting as a lens. Less protection by lack of melanin and absence of sebum and stratum granulosum may favor penetration of UV irradiation and explain why the skin was not always affected.