Objective To compare the differences of metabolic syndrome (MS) prevalence by using four working definitions and their relationship with obesity-related indicators in first-degree relatives of type 2 diabetes mellitus pedigrees. Methods Totally, 2 372 first-degree relatives from 715 type 2 diabetic pedigrees were selected in this study. Complete laboratory data, including blood pressure, lipid profile and plasma glucose, were collected. The prevalence rates of MS and obesity of four definitions, as defined by National Cholesterol Education Program Adult Treatment Panel Ⅲ (ATPⅢ) in 2005, International Diabetes Federation (IDF) in 2005,Chinese Diabetes Society (CDS) in 2004 aml Joint Committee for Developing Chinese Guidelines on Prevention and Treatment of Dyslipidemia in Adults (JCDCG) in 2007,were analyzed. Results (1)The prevalence rates of MS were 45.40% ,38.74% ,25.08% and 39.29% aecording to four definitions respectively. The prevalence rates of MS were higher in females than in males by using ATPⅢ and IDF definitions (both P<0. 01). (2)The common comhinations of metabolic abnormality was dyslipidemia, hypertension, obesity and hyperglycemia by using four definitions,except in females by using CDS definition. (3)The prevalence rates of obesity were 58.18% ,58.18% ,33.90% and 42.96% acconling to the four definitions respectively. The prevalence rates of MS in obese subjects were 66.59% ,66.59% ,54.85% and 68.99% according to four definitions respectively. (4) Applying the cutoff point for abdominal obesity according to ATPⅢ, IDF and JCDCG definitions, the prevalence rates of abdominal obesity in subjects with body mass index (BMI) <25 kg/m2 were respectively 28.58% and 16.78%, being higher in females than in males(38.90% vs 15.02% ,21.01% vs 11.22% ,both P<0. 01). Conclusion (1)There is significant familial aggregation of MS and obesity,and the first-degree relatives of type 2 diabetic patients are high risk populations. (2) Waist circumference rather than BMI taken as a discriminating component of obesity in MS seems to be clinically more helpful to the early identification and prevention of MS.