Although developmental delays are not uncommon in children, the incidence is seldom assessed, and the reported prevalence varies widely. In Taiwan, the government mandates the reporting of suspected cases. Using the national registry data, we conducted a study to estimate the incidence and prevalence of developmental delays in young children in Taiwan and to assess the gender and geographic differences. According to the law, each city and county in Taiwan needs to establish a reporting and referral center. The Department of Interior constructed a surveillance system on the basis of these centers and publishes the registry data annually. We analyzed the data from 2003 to 2008. From 2003 to 2008, 73,084 new cases were registered, and the incidence was 5.7-11.1 per 1000 person-year under 3 years of age and 7.9-11.4 per 1000 person-year at 3-5 years of age. The estimated prevalence was 8.6-16.6 per 1000 under 3 years of age and 26.2-47.6 per 1000 at 3-5 years of age. The average age at reporting decreased from 3.4 years in 2003 to 3.1 years in 2008. In addition, we found that boys had higher incidence than girls all through the years. Rural areas had higher incidence than urban areas, except for 2003. (Contains 5 tables.)