Objectives: To compare the validity of parent-reported height, weight and body mass index (BMI) values of children (aged 4-10 years), when measured at home by means of newly developed instruction leaflets in comparison with simple estimated parental reports. Design: Randomised controlled trial with control and intervention group using simple randomisation. Setting: Belgian children and their parents recruited via schools (multistage cluster sampling design). Participants: 164 Belgian children (53% male; participation rate 62%). Intervention: Parents completed a questionnaire including questions about the height and weight of their child. Parents in the intervention group received instruction leaflets to measure their child's weight and height. Classes were randomly allocated to the intervention and control groups. Nurses measured height and weight following standardised procedures up to 2 weeks after parental reports. Outcome measures: Weight, height and BMI category of the child were derived from the index measurements and the parental reports. Results: Mean parent-reported weight was slightly more underestimated in the intervention group than in the control group relative to the index weights. However, for all three parameters (weight, height and BMI), correlations between parental reports and nurse measurements were higher in the intervention group. Sensitivity for underweight and overweight/obesity was respectively, 75% and 60% in the intervention group, and 67% and 43% in the control group. Weighed κ for classifying children in the correct BMI category was 0.30 in the control group and was 0.51 in the intervention group. Conclusions: Although mean parent-reported weight was slightly more underestimated in the intervention than in the control group, correlations were higher and there was considerably less misclassification into valid BMI categories for the intervention group. This pattern suggests that most of the parental deviations from the index measurements were probably due to random errors of measurement and that diagnostic measures could improve by encouraging parents to measure their children's weight and height at home by means of instruction leaflets.