CONTEXT: Elevated C-reactive protein (CRP) is a marker of cardiovascular risk in adults. Patterns and determinants of CRP in adolescents have not been well described. OBJECTIVE: This study aimed to determine how CRP varies by age, gender, Tanner stage, and body fat composition in rural Chinese adolescents and to what degree adiposity-CRP associations are attributable to shared genetic and environmental factors. DESIGN AND SETTING: Data were derived from an ongoing study of metabolic syndrome in a large community-based twin cohort enrolled in Anqing, China. PARTICIPANTS: The study sample included 1180 adolescent twins aged 13-21 yr. MAIN OUTCOME MEASURES: Plasma CRP concentrations were measured by sandwich immunoassay using flow metric xMAP technology. Body fat composition was assessed by dual-energy x-ray absorptiometry. RESULTS: CRP levels linearly increased across age and Tanner stage in males (P = 0.0001), but in females, CRP exhibited no trend after adjusting for fat mass (P > 0.05). For males, the most explanatory measure was body mass index (partial r(2) = 5.2%), whereas percent body fat (partial r(2) = 8.8%) was more explanatory in females. Of the phenotypic correlations between adiposity measures and CRP (0.25-0.28), 86-89% were attributed to shared genetic factors and 11-14% to common unique environmental factors in both sexes. CONCLUSIONS: Adiposity is a strong determinant of CRP even in this relatively lean Chinese population. There is notable gender difference for the CRP pattern and the relationship of CRP with adiposity during adolescence. To a large degree, common genetic factors may underlie the observed adiposity-CRP-phenotypic correlations.