To the Editor.— It is well known that patients who use fluorouracil topically may get an intensification of the reaction when they are exposed to sunlight. To take advantage of this, a series of 100 patients were given an erythema dose of ultraviolet light to the right half of the face on the day prior to using topical 1% fluorouracil in Propylene Glycol. The fluorouracil was applied to the entire face so that a contrast between the two sides could be made. An artificial source was selected so that a definite erythema dose could be given and it could be given previous to the use of the topical medication so that the possibility of photosensitization from the medication would be reduced or eliminated. Within a day, erythema from the ultraviolet light appeared on the right half of the face. As this faded over the next day or two, it became