Aim: The aim of this study is to investigate the differences between reported and measured weight and height for underweight, normal-weight, and overweight children, particularly in a general practitioner setting. Background: Screening, signaling, and treatment of childhood obesity by the general practitioner depends on accurate weight and height measurements.Methods: Data on reported and measured weight and height from a cohort including 715 normal-weight and overweight children aged 2–17 were used. Means of reported and measured weight and height were compared using the paired T-test. Findings: Of the 715 included children, 17.5% were defined as being underweight, 63.2% normal-weight, and 19.3% overweight according to direct measured height and weight. In the age group 2–8 years, parents of underweight children reported a significantly higher weight than measured weight [mean differences (MD) 0.32 kg (0.02, 0.62)], whereas parents of overweight young children reported a significantly lower weight [MD −1.08 kg (−1.77, −0.39)]. In the age group 9–17 years, normal-weight [MD −0.51 kg (−0.79, −0.23)] and overweight children [MD −1.28 kg (−2.08, −0.47)] reported a significantly lower weight than measured weight. Conclusions: General practitioners cannot rely on reported weight and height measures from parents and children. In case of suspected under- or overweight in children, it should be advised to measure weight and height in general practice.