The incidence and clinical features of allergic contact and/or photocontact dermatitis due to psoralens were examined in 371 patients with psoriasis treated with topical PUVA. The psoralen derivatives used in the study were 8-methoxypsoralen (8MOP), 3-carbethoxypsoralen (3CPs), 4,6,4'-trimethylangelicin (TMA) and 7-methyl pyridopsoralen (MPP). Of 371 patients treated with 8MOP, three (0.8%) developed an acute dermatitis in the PUVA-treated areas. This incidence was significantly lower (P less than 0.01) than that for 3CPs (four of 10 patients) or that for TMA (six of 17 patients). None of the seven patients receiving MPP on PUVA had a reaction. It was confirmed that these dermatitis reactions were due to contact and/or photocontact allergy to psoralens by several methods that include patch and photopatch tests, photopatch test mapping, determination of the minimal erythema dose (MED) and immunohistochemistry.