Parent-reported height and weight are often used to estimate BMI and overweight status among children. The quality of parent-reported data has not been compared to measured data on a national scale for all race/ethnic groups in the United States. Parent-reported height and weight for 2-17-year-old children in two national health interview surveys--the 1999-2004 National Health Interview Survey (NHIS) and the 2003-2004 National Survey of Children's Health (NSCH)--were compared to measured values from a national examination survey-the 1999-2004 National Health and Nutrition Examination Survey (NHANES). Compared to measured data, parent-reported data overestimated childhood overweight in both interview surveys. For example, overweight prevalence among 2-17-year-olds was 25% (s.e. 0.2) using parent-reported NHIS data vs. 16% (s.e. 0.6) using measured NHANES data. Parent-reported data overestimated overweight among younger children, but underestimated overweight among older children. The discrepancy between reported and measured estimates arose mainly from reported height among very young children. For children aged 2-11 years, the mean reported height from NHIS was 3-6 cm less than mean measured height from NHANES (P < 0.001) vs. no difference among children aged 16-17 years. Measured data remains the gold standard for surveillance of childhood overweight. Although this analysis compared mean values from survey populations rather than parent-reported and measured data for individuals, the results from nationally representative data reinforce previous recommendations based on small samples that parent-reported data should not be used to estimate overweight prevalence among preschool and elementary school-aged children.