Objective: To investigate the prevalence of low body weight in Beijing pubertal girls and to establish the cut-off for body mass index (BMI) for underweight for Chinese pubertal girls. Design: Cross-sectional study. Setting: Three socioeconomic areas (rural, suburban and urban) in Beijing, China. Subjects: Random sample of 1214 adolescent girls aged 12-14y from 13 middle schools. Results: Using a modified Chinese reference, the rate of low body weight (BMI < 18) was 32.2% (95% CI 29.6-34.8%). Compared with desirable weight girls (BMI = 18-21), girls with low body weight had a lower bone age, delayed breast and pubic hair development, a lower rate of menarche, lower distal one-third radius and ulna bone mineral content (BMC), bone mineral density and bone width. Logistic regression showed that BMI was one of the predictors of one-third ulna BMC after adjustment for confounding variables. When comparing BMI < 18 vs BMI = 18-21, the risk of BMC being less than the median increased by 82% (odds ratio 1.82, 95% CI 1.06-3.13). Thinness and stunting rates assessed by WHO recommended cut-offs are also reported. Conclusions: High prevalence of low body weight (BMI < 18) was found to be a major health problem among Beijing pubertal girls. BMI < 18 is confirmed as the cut-off for delayed general growth arid development for Chinese girls and for screening girls at risk of lower bone mineral status. Sponsorship: The research was supported in part by the Dairy Research and Development Corporation, Australia.