During clinical trials with benoxaprofen, some patients noted burning and stinging in the skin when exposed to light and some developed onycholysis. A four-part prospective study was undertaken. During the first part of the study it was demonstrated that (1) benoxaprofen is associated with a hypersensitivity to long wave-length ultraviolet light (UVA). During the remaining three parts of the study, patients were exposed to very high doses of UVA light in order to try to induce a photosensitivity response. These studies demonstrated that (2) the symptoms of burning and stinging in the skin and signs of erythema and induration after very high-dose UVA exposure (30 Joule) may be prevented by the prophylactic application of a factor 15 sunscreen; (3) exposure to sunlight is required for the development of onycholysis in patients on benoxaprofen; and (4) the development of onycholysis was prevented by the regular use of a nail polish containing sunscreen. A commercially available, colored, opaque nail polish also would be expected to provide protection from onycholysis.