The incidence of obesity has increased enormously in the past several decades, and has been described as a modern epidemic. Obesity is a major factor contributing to hypertension. To the best of our knowledge, no study of ambulatory blood pressure monitoring (ABPM) comparing men with women in relation to body mass indexes (BMI) has been performed. From December 2002 to May 2006, we performed 24-h ABPM in 5950 subjects (3102 men and 2848 women), with a wide range of BMI (range 15.9-53.2 kg/m2). We defined obese subjects as those with BMI ≥30.0 kg/m2, overweight subjects as those with BMI >25.0 and <30.0 kg/m2, and normal subjects as those with BMI ≤25.0 kg/m2. Data on 989 subjects (501 men and 488 women) aged from ≥18 to ≤69 years without antihypertensive treatment, atrial fibrillation or diabetes were included for analysis. We consistently found that obese men had the expected increased heart rate compared to normal and overweight men, whereas women (normal, overweight and obese) had similar HRs. In addition, normal and obese women had similar diastolic blood pressures (BP), as opposed to obese men, who had raised diastolic BP. These results may indicate that different pathogenetic mechanisms may be involved in the relationship between obesity and hypertension in men and women.