The object of this study was to investigate the epidemiologic pattern of autism spectrum disorder in Chinese children. An autism spectrum disorder registry has been established in Hong Kong since 1986 by collecting data in a single center (the only university-affiliated child assessment center in Hong Kong). Since 1997, inpatient data from all public hospitals under the Hospital Authority have been stored in a central computerized program and retrieved from the Clinical Data Analysis and Reporting System. Clinical data have also been retrieved through the Clinical Data Analysis and Reporting System to ensure the completion of the registry, and these suspected cases have undergone the same diagnostic evaluation for autism spectrum disorder, as some of the new autism cases might be hospitalized in the public hospital. The incidence and prevalence of autism spectrum disorder have been calculated for the period of 1986 to 2005 using the population statistics available in the government for children less than 15 years old in Hong Kong. This study has included 4 247 206 person-years from 1986 to 2005 for children less than 15 years old and 1 174 322 person-years for those less than 5 years old in Hong Kong. Altogether, 645 children 0 to 4 years old with diagnoses of autism spectrum disorder were identified from 1986 to 2005. The estimated incidence of autism spectrum disorder was 5.49 per 10 000. The prevalence was 16.1 per 10 000 for children less than 15 years old for the same period. The male to female ratio was 6.58:1. This is the first reported epidemiologic study of autism spectrum disorder in Chinese children. The incidence rate is similar to those reported in Australia and North America and is lower than Europeans.