Aim: To construct the body mass index reference curves for Turkish children aged 6 to 18 y, and to determine the prevalence of overweight and obesity. Subjects and methods: Height and weight measurements of healthy schoolchildren (1100 boys, 1019 girls) were obtained biannually. Body mass index was calculated from 11648 height and weight measurements. Centile curves were constructed using the LMS method. Results: The smoothed percentile values and curves for body mass index in Turkish children show that there is a constant increase in body mass index values towards adulthood, especially during the pubertal years, in both sexes. The prevalence of overweight is 25% and obesity 4% in boys at age 18 y. The percentage of overweight and obesity in girls at age 14 was 15% and 1%, respectively. The sample size was too small to come to any conclusion regarding these rates at 18 y of age.