Skin biopsy specimens of patients with psoriasis receiving psoralen/ultraviolet A treatment were examined. A decrease in tonofilaments and keratohyaline granules and enlarged intercellular spaces were common findings in all cases. Hyperchromatic nuclei in the horny cells and hyperactive keratinocytes in the malpighian layer were also observed. Treatment resulted in normalization of keratinocytes and stimulation of melanocytes. Our study revealed that psoralen/ultraviolet A is effective and important in the treatment of psoriasis, but patients should be evaluated for atypical changes.