Electron microscopy of psoriatic skin prior to and after local hyperthermia revealed both temporary and gradual changes following treatment. The former occurred almost simultaneously with initiation of hyperthermia, and were characterized by an intracellular edema, the aggregation of tonofibrils in the basal half of the epidermis and an amorphous substance in the epidermal interspaces. The latter occurred during the course of treatment and were represented by narrowing of the intercellular spaces, a decrease in the number of mitochondria and free ribosomes, and an increased number of desmosomes and tonofibrils, with a consequent increase of keratohyaline granules. No such abnormal cells and structures were found in this study, as were observed in PUVA therapy.