FIELD : medicine; pharmaceuticals.SUBSTANCE : invention refers to medical chemistry, pharmaceutics and dermatology and can be used for phototherapy of psoriasis and psoriatic arthritis. Disclosed is the use of N-(6, 8, 8-trimethyl-8, 9-dihydrofuro[3, 2-h]quinoline-5-yl)acetamide as a skin-acting photosensitizer for treating psoriasis and psoriatic arthritis by PUVA therapy.EFFECT : use of N-(6, 8, 8-trimethyl-8, 9-dihydrofuro[3, 2-h]quinolin-5-yl)acetamide in combination with irradiation with long-wave ultraviolet light reduced skin hyperkeratosis and lymphocyte content of Th17-type and production of IL-17 and IL-23 lymphokines (key biological markers of psoriatic skin lesions) with efficacy not lower than authorized ammifurin preparation with low toxicity in intragastric and ocular administration; in contrast to ammifurin it is effective both at subminimal, and at minimum phototoxic doses.3 cl, 4 dwg, 8 tbl