In a group of 40 patients suffering from wide-spread psoriasis oral administration of a retinoid (Ro 10-9359) was followed by PUVA therapy. The clearance rate was increased by 30% as compared to PUVA alone. Except for cheilitis no side effects were seen. Histological analysis in 20 patients before, during and after therapy revealed an intensification of psoriatic tissue changes after retinoid treatment. Loss of corneal layers, massive exoserosis, and neutrophil migration were prominent features. Mitotic counts were not increased by the pretreatment. The increased susceptibility of diseased skin to PUVA as produced by this drug appears to be based on several factors related to the tissue changes revealed by histology.