There is evidence that reactive oxygen species and free radicals may be involved in the pathogenesis of photosensitivity in erythropoietic protoporphyria (EPP). Considering the well-known antioxidant properties of vitamin C, we investigated whether oral supplementation with this vitamin was photoprotective in patients with EPP. Twelve patients with EPP received either oral vitamin C 1 g daily or placebo, for 4 weeks, followed by a crossover period of another 4 weeks. Nine patients were already receiving beta carotene at entry and continued this at the same dose throughout the study. Patients compared their sunlight tolerance throughout each of the treatment periods with sunlight tolerance at entry on a 10 cm visual analogue scale; at the end of the study, they were asked to choose which treatment period they felt had been associated with least photosensitivity. Eight patients stated that they were able to tolerate sunshine better during the vitamin C period, 2 during the placebo period and 2 noticed no difference between the two periods. This distribution of preferences approached but did not reach statistical significance in favour of vitamin C. Visual analogue scores improved by a median of 1.2 cm in the vitamin C period. This change too approached but did not reach statistical significance. Although these results do not reach statistical significance, it appears possible that oral vitamin C may reduce photosensitivity in some patients with EPP. A larger study is necessary to confirm this impression.