The antimetabolite, 5-fluorouracil (5-FU), produces in the skin redness, erosive lesions, and hyperpigmentation after intravenous use and from topical application.1-4There has been no note of nail changes except for brief mention of rare hyperpigmentation and transverse sulci after intravenous use. In the course of our studies of the past year with topical 5-FU for the treatment of warts (308 patients) and of skin malignancy (23 patients),5we have observed 14 cases of nail disturbances. All these occurred in the treatment of 39 patients with periungual warts. 5-Fluorouracil was used as 5% in Neobase,*5% with 5% salicylic acid in Neobase, 20% in Neobase, and 20% with 5% salicylic acid in Neobase. All of these were used under adhesive. Controls in these experiments were 5% salicylic acid in Neobase and adhesive alone. Adhesive plaster with salicylic acid was not used as a control. Salicylic acid was used to