['Japan lacks a population-based registration system for cerebral palsy (CP), therefore the nationwide prevalence of CP is unknown. Our aim was to estimate the number of children with CP using the National Database of Health Insurance Claims and Specific Health Checkups of Japan, which has been recently developed by the government.', 'Study participants were children and adolescents aged below 20 years, who had been assigned CP diagnosis codes more than once in claims issued between June 2012 and May 2013 from all health insurance schemes in Japan, except for Social Welfare and Elderly Health Insurance.', 'The number of participants with diagnosed CP was 44 381. The number of males with CP (25 237) was greater than the number of females (19 144). Peak CP prevalence per 1000 population was 2.39 at age 4 years, and this gradually declined with age. The prevalence of CP per 1000 population was 2.27 at age 5 to 9 years. The numbers of inpatients and outpatients with CP were 9126 (20.6%) and 35 255 (79.4%) respectively.'] Does estimation of the number of children with cerebral palsy using nationwide health insurance claim data in Japan?