To obtain reference values of the waist circumference in Spanish children, and to investigate their dependence on age and gender. Cross-sectional study. General school-age population. A representative sample of the schools in Zaragoza, Spain, was drawn from seven schools. The population selected comprised 1728 children with ages ranging from 6.0-14.9y. Of the original sample, 368 children (21.29%) were excluded because of chronic diseases or refusal. Finally, 1360 children and adolescents: 701 boys and 659 girls, were studied. Waist and hip circumferences were measured with an unelastic tape. Waist circumference tended to be higher in males than in females and this difference was significant after 11.5y. In general, hip circumference was higher in females than in males (statistically significant differences at 7.5, 10.5, 12.5 and 13.5 y). In general, percentile values of waist circumference were higher in males than in females, especially after 12.5 y. Difference between males and females on percentile 95 at 14.5 y was 7.6 cm. Hip was greater than waist in both sexes, and the two curves run nearly parallel in males. In females, while hip enlarges continuously, waist shows the reverse tendency between 11.5 and 14.5 y. Waist circumference showed higher values in boys than in girls, especially after 11.5 y, and waist values increase with age both in males and females. These findings justify the use of age and gender specific reference standards.