AbstractObjectiveTo assess the reliability and validity of body weight (BW) and body image (BI) perception reported by parents (in children) and by adolescents in a South American population.DesignCross-sectional study. BW perception was evaluated by the question, ‘Do you think you/your child are/is: severely wasted, wasted, normal weight, overweight, obese?’ BI perception was evaluated using the Gardner scale. To evaluate reliability, BW and BI perceptions were reported twice, two weeks apart. To evaluate validity, the BW and BI perceptions were compared with WHO BMI Z-scores. Kappa and Kendall’s tau-c coefficients were obtained.SettingPublic and private schools and high schools from six countries of South America (Argentina, Peru, Colombia, Uruguay, Chile, Brazil).ParticipantsChildren aged 3–10 years (n 635) and adolescents aged 11–17 years (n 400).ResultsReliability of BW perception was fair in children’s parents (κ=0·337) and substantial in adolescents (κ=0·709). Validity of BW perception was slight in children’s parents (κ=0·176) and fair in adolescents (κ=0·268). When evaluating BI, most children were perceived by parents as having lower weight. Reliability of BI perception was slight in children’s parents (κ=0·124) and moderate in adolescents (κ=0·599). Validity of BI perception was poor in children’s parents (κ=−0·018) and slight in adolescents (κ=0·023).ConclusionsReliability of BW and BI perceptions was higher in adolescents than in children’s parents. Validity of BW perception was good among the parents of the children and adolescents with underweight and normal weight.