Randomized controlled trials (RCTs) are criticized for the questionable generalizability of their patient samples to real world populations. This study compared the demographic and clinical characteristics of children from the Multimodal Treatment Study of Children with ADHD (MTA) to those from the NIMH Methods for the Epidemiology of Child and Adolescent Mental Disorders (MECA) Study and the National Health and Nutrition Examination Survey (NHANES). On the whole, we found no compelling evidence against the representativeness of the MTA sample, but the limited power from MECA and NHANES may have lead to noteworthy differences between datasets being statistically nonsignificant.