Surveillance and epidemiologic research on intellectual disability often do not incorporate adaptive functioning (AF) data. Exclusion of AF data leads to overestimation of the prevalence of intellectual disability, the extent of which is not known. In this study, the authors evaluated the effect of incorporating AF data on overall intellectual disability prevalence according to sociodemographic, economic, and severity characteristics. Between 2002 and 2006, the Metropolitan Atlanta Developmental Disabilities Surveillance Program identified 1,595 8-year-old children who met the study's intellectual disability surveillance-case definition of IQ ≤ 70. AF scores were not available for 9.2% of the case children, specifically those with mild intellectual disability and low socioeconomic backgrounds. Prevalence estimates showed few substantive changes when incorporating AF data. The authors conclude that use of IQ data alone appears to be appropriate for measuring population intellectual disability prevalence.