Purpose: Obtaining stature, weight, and body mass index (BMI) from self-reports rather than by direct measurements is highly attractive economically and logistically, but there are few data available for adolescents that allow evaluation of potential sources of reporting bias. Because self-reports are based on self perceptions, personal characteristics of youth may be related to errors in reporting. Methods: Differences between self-reported and measured stature, weight, and BMI were investigated for a sample of 3797 Minnesota youth, 12 to 18 years of age. Gender, age, race/ethnicity, socioeconomic status (SES), and measured body size were examined as potential factors associated with errors in self-reports. Results: Self-reported stature, weight, and BMI were generally highly correlated with corresponding measured dimensions, although adolescents of both genders systematically overestimated their statures, underestimated their weights, and underestimated BMI. Significant associations of errors in self-reports with age, race/ethnicity, and socioeconomic status suggested that differences in self perceptions or value ascribed to body size varies according to these personal characteristics. Systematic errors in self-reported stature, weight, and BMI were negatively associated with the corresponding measured dimension when adjusted for age, race/ethnicity and SES, such that prevalences of overweight based on BMI from self-reported measures were systematically underestimated relative to measured values. Conclusions: Self-reports of stature, weight, and BMI are on the average, valid representations of their measured counterparts; nevertheless, errors in the self-reports are systematically related to characteristics of youth. Consequently, findings from these studies should be interpreted carefully.