The MMWR report presents findings from the Autism and Developmental Disabilities Monitoring (ADDM) Network, which collected data on autism spectrum disorder (ASD) prevalence in six U.S. sites in 2000. The study identified 1,252 children aged 8 years with ASD, with prevalence ranging from 4.5 to 9.9 per 1,000 children. The overall average prevalence was 6.7 per 1,000 children, indicating that ASDs are more common than previously believed. The male-to-female ratio varied from 2.8:1.0 to 5.5:1.0. Most children with ASDs had documented developmental concerns before age 3 years and received special education services. Cognitive impairment was reported for 40%–62% of children with ASDs. The study highlights the importance of using multiple data sources to estimate true prevalence and underscores the need for continued surveillance to track changes in ASD prevalence. The ADDM Network's methodology, which uses multiple data sources and standardized procedures, provides a reliable estimate of ASD prevalence. The study also notes that prevalence varied by race/ethnicity and site, with New Jersey having the highest prevalence. The findings emphasize the importance of early identification and intervention for children with ASDs and highlight the need for further research to understand the factors contributing to differences in ASD prevalence across sites. The study also acknowledges the limitations of using records-based data and the potential for underestimation of prevalence due to incomplete or missing records. Overall, the study provides important baseline data for understanding ASD prevalence in the U.S. and underscores the need for ongoing surveillance and research to better understand and address the public health impact of ASDs.The MMWR report presents findings from the Autism and Developmental Disabilities Monitoring (ADDM) Network, which collected data on autism spectrum disorder (ASD) prevalence in six U.S. sites in 2000. The study identified 1,252 children aged 8 years with ASD, with prevalence ranging from 4.5 to 9.9 per 1,000 children. The overall average prevalence was 6.7 per 1,000 children, indicating that ASDs are more common than previously believed. The male-to-female ratio varied from 2.8:1.0 to 5.5:1.0. Most children with ASDs had documented developmental concerns before age 3 years and received special education services. Cognitive impairment was reported for 40%–62% of children with ASDs. The study highlights the importance of using multiple data sources to estimate true prevalence and underscores the need for continued surveillance to track changes in ASD prevalence. The ADDM Network's methodology, which uses multiple data sources and standardized procedures, provides a reliable estimate of ASD prevalence. The study also notes that prevalence varied by race/ethnicity and site, with New Jersey having the highest prevalence. The findings emphasize the importance of early identification and intervention for children with ASDs and highlight the need for further research to understand the factors contributing to differences in ASD prevalence across sites. The study also acknowledges the limitations of using records-based data and the potential for underestimation of prevalence due to incomplete or missing records. Overall, the study provides important baseline data for understanding ASD prevalence in the U.S. and underscores the need for ongoing surveillance and research to better understand and address the public health impact of ASDs.