BACKGROUND: This study was conducted to assess the national prevalence of different grades of nutritional status (underweight, normal weight, overweight and obesity) among Iranian school-students and to compare the prevalence of overweight and obesity using three different sets of criteria. METHODS: This cross-sectional national survey was conducted on a representative sample of 21 111 school students including 10 253 boys (48.6%) and 10 858 girls (51.4%) aged 6-18 years, selected by multistage random cluster sampling from urban (84.6%) and rural (15.4%) areas of 23 provinces in Iran The percentage of subjects in the corresponding body mass index (BMI) categories of the Centers of Disease Control and Prevention (CDC), the International Obesity Task Force (IOTF) and the obtained national percentiles were assessed and compared. RESULTS: There was no gender differences in BMI, but was higher in boys living in urban than in rural areas (18.4 +/- 3.88 vs. 17.86 +/- 3.66 kg/m(2) respectively, P < 0.05). The prevalence of underweight was 13.9% (8.1% of boys and 5.7% of girls) according to the CDC percentiles, and 5% (2.6% of boys and 2.4% of girls) according to the obtained percentiles. According to the CDC, IOTF and national cut-offs, the prevalence of overweight was 8.82%, 11.3% and 10.1% respectively; and the prevalence of obesity was 4.5%, 2.9% and 4.79% respectively. The prevalence of overweight was highest (10.98%) in the 12-year-old group and that of obesity (7.81%) in the 6-year-old group. The kappa correlation coefficient was 0.71 between the CDC and IOTF criteria, 0.64 between IOTF and national cut-offs, and 0.77 between CDC and national cut-offs. CONCLUSIONS: The findings of this study warrant the necessity of paying special attention to monitoring of the time trends in child obesity based on uniform definitions, as well as to design programmes to prevent and control associated factors.