A 61-year-old female patient with xeroderma pigmentosum (XP), registered as XP46KO, was assigned to complementation group F by the cell fusion-complementation method. The XP46KO fibroblasts in culture exhibited a defective DNA repair capacity of 10-15% unscheduled DNA synthesis and a 3-fold sensitivity to the lethal effect of 254 nm ultraviolet light compared with normal cells. The patient had mild clinical symptoms consisting of numerous pigmented freckles and a small number of seborrheic keratosis-like papules. She had no skin cancers in the sun-exposed areas of the skin and so far no neurological abnormalities. A review of 11 Japanese group F patients revealed very mild skin symptoms with no ocular or neuro-psychiatric abnormalities. Single skin cancers occurred in only 3 of the 11 patients with an average age of 52 years for their first skin malignancy.