The prevalence of autism spectrum disorders (ASD) in the world has increased dramatically in the recent decades. However, data at the national level are limited, and geographic differences are seldom evaluated. According to the law, the local governments in Taiwan began to certify disabled residents and provide various services in 1980, and the central government maintains a registry of certified cases. The registry started to enroll cases of ASD in 1990, providing a unique opportunity for studying ASD at the national level. Because the government discourages the certification under 3 years of age, we limited our analyses to those who were at least 3 years old. Using the registry data from 2004 to 2010, we calculated the prevalence of ASD by age, gender, and geographic area and assessed the changes over time. From 2004 to 2010, the registered cases between 3 and 17 years old increased from 3995 to 8072 annually, and the prevalence generally increased every year in all age groups (p less than 0.01). In each year there were more boy cases than girl cases, and the prevalence rate ratio ranged from 5.64:1 to 6.06:1 (p less than 0.01 in all years), with an increasing trend over time (p less than 0.01). A higher prevalence was observed in the urban areas over the years, and the prevalence rate ratio ranged from 2.24:1 to 2.72:1 (p less than 0.01 in all years), with a decreasing trend over time (p less than 0.01). (Contains 3 tables.)