The sun's rays appear to be valuable in the treatment of psoriasis, and in most cases the condition improves during the summer months. In many cases of extensive lesions with indurated plaques and in cases in which the condition becomes aggravated rather than improved in the summer, it is possible that the skin is not photosensitive. In this paper I wish to present a method whereby, according to some experiments, photosensitivity can be increased. Goeckerman, in 19251 and again in 1931,2 reported success from the application of an ointment of coal tar followed twenty-four hours later by exposure to ultraviolet rays. Lane and Crawford,3 in their statistical study of 231 cases, reported improvement in over 60 per cent and exacerbation in about 14 per cent during the summer season. They secured the best results by treatment with external applications of sulfur, coal tar, chrysarobin, ammoniated mercury, sunlight