Nineteen children with hypomelanosis of Ito are described. Fourteen were developmentally delayed and nine had a history of seizures. Hemihypertrophy was present in four patients, syndactyly in three, and scoliosis in one. Twelve of the children had abnormal electroencephalograms and nine had abnormal brain scans, four with appearances suggestive of abnormal neuronal migration. There is very little evidence, either from the literature or from our patients, that the disease is inherited. The pattern of the cutaneous lesions suggests that the condition may result from the presence of two different cell populations as a result of mosaicism.