Xeroderma pigmentosum is characterized by sunlight sensitivity, multiple cutaneous malignancies, and autosomal recessive inheritance. Some patients also demonstrate neurological manifestations, ie, mental deficiency, microcephaly, ataxia, choreoathetosis, and deafness. Five patients were studied, four with neurological disease, one of whom also had lymphatic leukemia. Autopsy of one patient disclosed cerebral and olivopontocerebellar atrophy. There was nuclear atypia of pancreatic islet cells and liver cells similar to that seen in ataxia-telangiectasia. Normal skin fibroblasts can repair ultraviolet radiation damage to deoxyribonucleic acid by inserting new bases into DNA in the form of small patches. However, cells from patients with xeroderma pigmentosum carry a mutation such that repair replication of DNA is either absent or greatly reduced in comparison to normal fibroblasts. The failure of DNA repair in the skin may be related to carcinogenesis which occurs in sun-exposed areas of the skin.