OBJECTIVE To construct the body mass index (BMI) reference data and curves for Chinese children and adolescents from birth to 18 years of age. METHODS Data from two national representative cross-sectional surveys which were The National Growth Survey of Children under 7 years in the Nine Cities of China in 2005 and The Physical Fitness and Health Surveillance of Chinese School Students in 2005. Height (length was measured for children under 3 years) and weight data of 93,702 urban healthy children from nine cities/provinces used to calculate the BMI. The LMS method was used to smooth the BMI, with estimates of L, M, and S parameters, values of percentile and Z-score curves which were required were calculated, and then standardized growth charts were generated. Adult cut-offs for overweight and obesity at 18 years was used to study the cut-offs for children 2 to 18 years of age. RESULTS The smoothed percentiles and Z-scores reference data and curves of BMI-for-age for boys and girls aged 0-18 years were made out respectively. BMI cut-off values for overweight and obesity for children from 2 to 18 years of age were also defined. At 18 years, the BMI values are equivalent to the overweight cut-off (> or =24 kg/m2) and obesity cut-off (> or =28 kg/m2) for Chinese adults. Comparison with the reference of the WHO and 2000 CDC for the United States, there were some difference among them, at the 97th percentile curve there was a big difference between Chinese and U. S. adolescents. On the whole, the China BMI curve for boys was higher than the new WHO curve and lower than 2000 CDC at 97 percentile curve, but the China BMI curve for girls was lowest among the three curves. There was also significant difference between China and Japan BMI values at 97 percentile curve. CONCLUSIONS BMI growth curves are very useful in child growth monitoring and nutritional surveillance, discovering overweight and obesity. The BMI growth charts are recommended for use in pediatric clinic and public health service.