<i>Background:</i> The relationship between parent-reported and measured height and weight is not well established in schoolchildren. This relationship has never been studied in asthmatic children. The objective of this study is to test the validity of the parent-reported weight and height for defining obesity by BMI and to know whether the perception of this height and weight changes when the child suffers from asthma. <i>Methods:</i> All classes of children of the target ages of 6–8 years (n = 1,672, participation rate 70.2%) of all schools in four municipalities of Murcia (Spain) were included. Parents were asked about their children’s weight and height using a questionnaire which included the International Study of Asthma and Allergies in Childhood (ISAAC) core questions on asthma. Parents were not aware that their children were going to be weighed and measured within 1 week’s time. Measurements were performed using a rigid stadiometer to the nearest 0.1 cm and a scale to the nearest 0.1 kg. <i>Results:</i> The bias (reported minus real) was, respectively, for nonasthmatics and asthmatics: weight +0.42 kg (95% CI +0.24; +0.59 kg) versus +0.97 kg (+0.50; +1.44 kg), height +2.37 cm (+2.06; +2.68 cm) versus +2.87 cm (+1.87; +3.87 cm); BMI –0.39 kg/m² (–0.52; –0.23 kg/m²) versus –0.23 kg/m² (–0.58; +0.13 kg/m²). Diagnostic accuracy of obesity calculated from reported measurements was, respectively, for nonasthmatics and asthmatics: sensitivity 78.0 versus 77.8%, specificity 96.2 versus 94.5%, positive predictive value 77.2 versus 73.7% and negative predictive value 96.4 versus 91.7%. <i>Conclusions:</i> Reported weights and heights had large biases, comparable between parents of both asthmatic and those of nonasthmatic children. However, this information could be reasonably valid for classifying children as obese or nonobese in large epidemiological studies.