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Background Erythropoietic protoporphyria ( EPP) is a rare metabolic disorder, characterized by photosensitivity, caused by errors of the haem biosynthetic pathway. Avoidance of sun exposure is recommended; however, some patients suggested a paradoxical improvement of symptoms when they move to sunny areas. Objectives In a national French study, we sought to investigate the influence of sun exposure on EPP symptoms. Materials and methods We used a national transversal observational study by questionnaire. Patients were selected from the national record of the Centre Français des Porphyries (French Porphyrias referral centre). Sun exposure level by geographic area was assessed using climate data provided by the French national meteorological service (Météo France). Results Eighty-nine patients were included. We notably observed that 40% of patients declared an improvement in their tolerance of sun exposure after repeated sun exposures. In the more sunny areas, the intensity of the pain was lower ( r = −0·26) and the duration of the sun exposure responsible for flares was longer ( r = 0·39) than in the areas that were less sunny ( P < 0·05). Conclusions This study proposes a benefit of natural progressive sun exposure for patients with EPP. [ABSTRACT FROM AUTHOR]