A 6 year-old boy with de Sanctis-Cacchione syndrome is reported. This syndrome is characterized by the triad xeroderma pigmentosum (XP), mental deficiency and neurological disturbances. The patient's cells were assigned to genetic complementation group A by use of the cell fusion technique. DNA repair capacity as measured by unscheduled DNA synthesis (UDS) was drastically reduced to 7.5%, compared with 100% of the controls. The rate of sister chromatid exchange (SCE), an indicator of the hypermutability in XP, was clearly elevated after ultraviolet radiation of skin fibroblasts of the patient.