We report a case of Cockayne syndrome. A 6-year-old boy presented with a progeroid face, dwarfism, psychomotor retardation, skin photosensitivity and retinal pigmented degeneration. Neurological study disclosed slowed nerve conduction velocities and a brain CT showed calcification in the basal ganglia. Auditory brain stem evoked potential showed prolonged interpeak latency of wave I to wave V. Laboratory evaluation revealed mild liver dysfunction and peripheral eosinophilia. Fibroblast cultures from the patient and his family were exposed to ultraviolet (UV) light of 254 nm, ranging from 1 to 10 J/m2. Under 1 J/m2 irradiation, the surviving fraction of the fibroblasts from the patient, his mother, and a control subject were 40%, 50%, 90% respectively. If the fibroblasts of these subjects were exposed to 2 J/m2 and 3 J/m2 irradiation, the surviving fraction changed to 10%, 22%, 80% and 1.5%, 9%, 68%, respectively. However, fibroblasts from his sister and father showed the same surviving fraction as the control. The study showed that fibroblasts from the patient and his mother were extremely sensitive to UV light irradiation. We also study the concentration of the pyrimidine dimer of DNA in the patient and the control subject. Pyrimidine dimer showed no difference between the patient and the normal subject before and after 24-hour UV irradiation. These results suggest that the sensitivity to UV of Cockayne fibroblasts may be due to a ligase deficiency or to a replicon initiation disturbance in Cockayne cells.