The error in self-reported weight and height compared with measured weight and height was evaluated in a nationally representative sample of 11,284 adults aged 20-74 y from the second National Health and Nutrition Examination Survey of 1976-1980. Although weight and height were reported, on the average, with small errors, self-reported weight and height are unreliable in important population subgroups. Errors in self-reporting weight were directly related to a person's overweight status--bias and unreliability in self-report increased directly with the magnitude of overweight. Errors in self-reported weight were greater in overweight females than in overweight males. Race, age, and end-digit preference were ancillary predictors of reporting error in weight. Errors in self-reporting height were related to a person's age--bias and unreliability in self-reporting increased directly with age after age 45 y. Overweight status was also a predictor of reporting error in height.