Childhood obesity is an increasing problem both in the US and many other countries. The etiology of obesity is not well understood. The purpose of this study is to investigate the relative importance of genetic and environmental factors as determinants of attained weight, weight gain, and body mass index(kg/m2) in children. This analysis includes 200 pairs of monozygotic(MZ) twins and 99 pairs of like-sexed dizygotic (DZ) twins aged 8 to 19 years in Beijing. The initial survey was conducted in May, 1983, and the follow-up survey in May, 1985. Weight and height were measured according to the standard protocol. Information on social demographic characteristics and daily activity was obtained by a standard questionnaire. Twin zygosity was determined by four blood groups (ABO, MN, P, Rh) and saliva secretion type. MZ and DZ twins were similar in age and sex composition, social demographic characteristics, and daily activity. For attained weight, weight gain, and body mass index respectively, MZ twins had smaller mean within-pair difference (2.1 kg, 0.7 kg/yr, 0.8 kg/m2) than DZ twins (4.9 kg, 1.1 kg/yr, 1.4 kg/m2); MZ twins also had higher within-pair correlations (0.96, 0.86, and 0.91) than DZ twins (0.76, 0.64, 0.71). There was no significant difference between the two groups of twins in total variance of attained weight, weight gain, and body mass index, but the within-pair variance for MZ twins was significantly smaller than that for DZ twins (P <0.01). Five different formula were employed to estimate heritability (h2). The results are consistent in indicating a major genetic contribution to the attained weight (h2: 0.38-0.96), weight gain (h2: 0.43-0.86), and body mass index(h2: 0.41-0.91). This study underscores the need of a genetic and epidemiologic approach to understand the etiology of childhood obesity. Early identification of those children with genetic susceptibility to obesity and early intervention could have a significant impact on subsequent adult health.