The aim of this study was to evaluate the validity of parentally reported weight and height values for preschool-aged children (3-7 years of age) in Belgium and the accuracy of BMI classifications using those data. The subjects were 297 preschool-aged children (157 boys and 140 girls). A questionnaire with questions about height and weight was distributed to the children and completed by their parents. Nurses of the Centres for Pupils Counselling conducted anthropometric measurements by following standardized procedures. National and international age- and gender-specific BMI cutoff values were used to determine BMI categories. Mean differences between reported and measured values for weight (-0.57 kg) and BMI (-0.51 kg/m2) were statistically significant, although small. For height, the mean difference (0.14 cm) was not statistically significant. When national and international BMI cutoff values for overweight were used, sensitivity values were 47% and 44%, respectively, and specificity values were 94% and 95%. Among all children requiring nutritional advice on the basis of being overweight or obese, more than one half of the overweight children and >75% of the obese children would be missed with the use of parentally reported weight and height values. Conversely, 70% of the children being classified as underweight with the use of these reported data could be encouraged wrongly to gain weight. The bias of parent-reported BMI values was significantly greater when weight and height were both guessed, rather than being measured at home. Our results show the inaccuracy of parentally reported weight and height values in Belgium for classifying preschool-aged children into BMI categories. Therefore, accurate measurements of weight and height should be encouraged in studies in which BMI of children is a variable of interest.