A case of porphyria cutanea tarda with experimental light urticaria.

Photobiological tests were carried out on a 32-year-old man who suffered from porphyria cutanea tarda (PCT). The patient developed an immediate type of skin reaction with erythema and whealing following monochromatic irradiation at 400 nm, but did not have any abnormal immediate skin reaction after exposure to natural sunlight. Pre- or simultaneous irradiation with visible light, wavelength greater than 650 nm, suppressed the development of urticaria induced by 400 nm monochromatic radiation. On the basis of these findings and our previous observation of an inhibitory spectrum in two cases of solar urticaria, we suggest that there is also an inhibitory spectrum in PCT. This could explain the extremely low incidence of immediate erythematous or urticarial reactions in sun-exposed skin in these patients.