Photochemotherapy (PUVA) with oral administration of the photosensitizer 8-methoxypsoralen and irradiation with UVA light has become a proved method for the therapy of psoriasis. The treatment is based upon repeated PUVA exposures, which are monitored by exact dosimetry within the desired therapeutic ranges. The present study, which covers an observation period of 3 years, is engaged with the practicability of the PUVA method for the treatment of the various forms of psoriasis, mycosis fungoides, atopic eccema and lichen planus, and, on the other hand, with its side effects and potential long term hazards. Photochemotherapy was shown in clinical routine as an highly effective treatment, which lead to complete clinical remission in all diseases investigated. By maintenance treatment patients with psoriasis could be kept in remission even for long periods of time. Following the dosage parameters toxic side effects were rare and reversible. Histological, histochemical and ultrastructural investigations showed no alteration of skin under photochemotherapeutic conditions. Experimentally induced massive PUVA overdosage, however, led to damage of epidermis and corium and to long lasting benign changes of the melanin pigment system. To study potential long term side effects (hepatotoxicity of 8-methoxypsoralen, cataract formation, development of degenerative or hyperplastic skin changes and disturbances of the immunologic reactivity) laboratory investigations, histologic, ophthalmologic and immunologic studies have been performed. During an observation period of up to 3 years no evidence of long term side effects was found.