Psoriasis is a autosomal hereditary disease of the skin, which has been treated with a great number of possibly carcinogenic substances. No increase of maligonomies has been observed except for the tumors induced by arsenic. The question may be asked whether the psoriatic cell has additional or more effective mechanisms to eliminate onocogenic somatic mutations. In this study the capacity of the excision repair of circulating blood lymphocytes from 12 psoriatic patients has been investigated and compared to a control group of equal size. After irradiation with ultraviolet light, the lymphocytes are incubated with 3 H-thymidine and at various intervals the radioactivity in the cell material precipitated with perchloric acid is measured. It appears, that the excision reapir in lymphocytes from patients with psoriasis and normal persons is equal. In addition, the influence of hydroxyurea on the thymidine incorporation in lymphocytes has been studied.