Siblings of Cockayne syndrome were reported. Both of them had characteristic manifestation of Cockayne syndrome. A 4-year-old boy had been noted to have irritability and nystagmus from the 6th day of his life, and his elder sister, 8 years old, had been diagnosed as cerebral palsy at the age of 6 months. The elder sister lacked photosensitivity clinically. The proband was more severely affected than his elder sister on both cerebral CT-scan and electrophysiological studies. Reduced colony forming ability after ultraviolet exposure is currently considered to be the primary abnormality in Cockayne syndrome. The cells originating from the boy had moderately reduced colony forming ability after ultraviolet irradiation, whereas the cells from his elder sister showed mildly reduced colony forming ability. There may be some relationship between the severity of the symptoms and the colony forming ability. There is a broad spectrum of clinical manifestations in Cockayne syndrome. Further investigations are necessary to clarify the relationship between the cytological abnormalities of Cockayne syndrome and its neurological manifestations.