An electron microscopic study was performed to demonstrate the pathological changes induced by long term PUVA treatment in recalcitrant psoriasis. Three patients developed mottling (hyperpigmentation and hypopigmentation) during two to three years of treatment. Three different types of morphological changes were found: disarrangement of keratinocytes, clustering and stimulation of melanocytes and homogenization of papillary dermis. Furthermore, the superficial blood vessels were loaded with the same type of amorphous granular substance. These changes might be specific to PUVA treatment or they might occur only in patients with previous treatment with, e.g., arsenic, methotrexate, anthralin + UVB or a combination of these.