The prevalence of overweight and obesity among schoolchildren aged 6.5–11.5 years in Shiraz (southern Iran) are presented in this paper. The body mass index (BMI) percentiles of these children are compared with the Center for Disease Control and Prevention (CDC) reference data and with the Iranian standard. The data are based on a random multistage sample survey of 2397 healthy school attenders (1268 boys, 1129 girls) living in Shiraz, whose heights and weights were measured in the 2002–2003 academic year. Joint height and weight measurements were obtained for 2195 schoolchildren (91.6%), consisting of 1138 boys (89.7%) and 1057 girls (93.6%). A total of 77 boys (6.8%) and 40 girls (3.8%) were overweight, and the difference between them was significant (P = 0.001). However, obesity was significantly less prevalent in boys (3.3%) than in girls (6.1%) (P = 0.001). Our children’s median BMI lie almost on the 38th centile of the CDC reference data, whereas that of their counterparts born more than 10 years ago lay on the 20th centile of their American counterparts, showing the development of children’s obesity in a period of less than 15 years in Iran. A positive secular trend in BMI has been seen during the past decade in Iran, suggesting policymakers and health professionals should pay special attention to children’s health.