We investigate the relationship between changes in socioeconomic factors and the emerging coexistence of under and overweight among adults in China during 1991-2000. Our key questions are: (1) whether any socioeconomic factor explains both increasing overweight (Body Mass Index (BMI)less than or equal to 25 kg/m2) and remaining underweight (BMI greater than or equal to 18.5 kg/m2), (2) whether China's continuing economic growth leads to further increase in the prevalence of overweight, and (3) whether China's economic growth alone can lead to commensurate decrease in its remaining underweight. Based on the theoretical model in Lakdawalla and Philipson (2002), we focus on the effects of economic growth on weight through changes in income, job-related activity and food prices. We adopt a semiparametric technique and decompose changes in the BMI distribution into the effects of changes in selected socioeconomic factors. We find that changes in the pattern of job-related activity partly explain both increasing overweight and remaining underweight. Overall income growth contributes to decreasing both under and overweight. Decreasing food prices are one of main factors shifting Chinese population from underweight toward overweight. The effects of economic growth examined in this paper well-explain increasing overweight, and thus continuing economic growth is likely to lead to further increase in overweight rates. Our results also indicate that there exist unobserved factors that significantly counteract the downward effects of economic growth on underweight rates, and thus economic growth alone is unlikely to lead to commensurate decrease in remaining underweight. To reduce remaining underweight, more direct interventions (e.g., micronutrient supplementation) may be needed.