OBJECTIVES To examine the association between waist circumference and body mass index in patients with metabolic syndrome. METHODS Based on a large electronic medical record database, all adult patients were identified for metabolic syndrome using the guidelines by the National Cholesterol Education Program Adult Treatment Panel III (NCEP ATP III). The waist circumference (WC) was regressed as a linear function of body mass index (BMI) while controlling for individual age, gender, and identified latent classes of metabolic risk factors. RESULTS A total of 18,788 patients with metabolic syndrome were identified from the database. The mean (standard deviation) age is 57.2 (13.9), with 51.3% being females. The prevalence of abnormal triglycerides (TG; ≥150 mg/dL), high-density lipoprotein cholesterol (HDL-C; men: <40 mg/dL; women: <50 mg/dL), blood pressure (BP; systolic ≥130 mmHg, diastolic ≥85 mmHg, or drug treatment for hypertension), fasting plasma glucose (≥100 mg/dL or drug treatment for diabetes mellitus), and waist circumference (WC; men: ≥102cm; women: ≥88cm) was 79.6%, 74.3%, 96.4%, 69.2%, and 81.4%, respectively. The Pearson's correlation between WC and BMI is 0.68. In a linear regression model only controlling for age and gender, it is found that one unit increase in BMI is associated with 0.68 inches more in WC (p<0.001). BMI, age and gender together explains 54.5% of the variation in WC. The positive relationship between WC and BMI still remains highly significant and changes little numerically, after including five latent classes of metabolic risk conditions. CONCLUSIONS For patients with metabolic syndrome, there is a strong and positive association between waist circumference and body mass index, which remains robust after controlling for unobserved metabolic risk conditions.