Objective: To determine the applicability of the World Health Organization growth curves in school children that live in areas of moderate altitude. Methods: A cross-sectional study, using a stratified random sample of 955 children and adolescents aged 6 to 12 years-old (473 boys and 482 girls) attending public schools in the urban area of Arequipa (Peru). The evaluated variables included body mass (kg) and height (m) values and the body mass index. Z score and the t test for paired measurements were applied for statistical analysis. Results: The boys had similar values of body mass when compared with the reference. However, girls showed higher values than the reference at ages six, seven, and ten years-old (p<0.001). For height and body mass index, differences (p<0.001) were noted between the reference and the school children living in areas of moderate altitude for all ages and genders, being the height lower than the reference and the body mass index higher than the reference. Z scores for the body mass index were: boys - 1.0 (six yearsold), 0.69 (seven years-old), 0.50 (eight years-old), 1.20 (nine years-old), 0.75 (ten years-old), 0.41 (11 years-old) and 0.82 (12 years-old); girls - 0.36 (six years-old), 0.53 (seven and eight years-old), 0.48 (nine years-old), 0.89 (10 yearsold), 0.55 (11 years-old), and 0.43 (12 years-old). Conclusions: The body mass index should not be applied to children and adolescents that live in regions of moderate altitude. The linear growth retardation presented by this population directly compromises the results of this index.