The aim of this study was to determine the prevalence of overweight and obese subjects in the Shanghai population of China and its association with undiagnosed hypertension, by taking age, gender and place of residence (urban or suburban) into account. A cross-sectional population-based survey was conducted in 2007. The sample included 13,359 participants aged 15-69 years.Weight, height, and blood pressure were recorded, and information about gender, age and place of residence was obtained. Overweight and obesity prevalence were calculated by the body mass index (BMI) definition recommended by Working Group on Obesity in China (normal weight, 18.5-23.9 kg/m(2); overweight, 24-27.9 kg/m(2); obesity, ≥ 28 kg/m(2)). Undiagnosed hypertension was defined by China criteria in accord with that of WHO-ISH (subjects with systolic pressure ≥ 140 mmHg, and/or diastolic pressure ≥ 90 mmHg). Multiple logistic regression analyses were used to assess the association of overweight or obesity with undiagnosed hypertension by adjusting for age, gender and place of residence. The overall overweight, obesity, and undiagnosed hypertension prevalence were 27.6% (95% CI: 26.8-28.4), 6.6% (95% CI: 6.2-7.0), and 15.5% (95% CI: 14.9-16.1), respectively. Compared to normal weight subjects, the odds ratios (OR) for subjects who were overweight and had hypertension was 2.33 (95% CI: 2.10-2.59); that for obesity and hypertension was 4.27 (95% CI: 3.66-4.99). These data suggest that overweight and obesity prevalence and their association with undiagnosed hypertension are high in our study population.