It has been shown that aging and hypertension are important risk factors to promote renal damage. However, little data are available on the effect of obesity on the progression of renal damage, especially in young and middle-aged individuals. The aim of this study was to determine the association between body mass index (BMI) and renal function evaluated by estimated glomerular filtration rate (eGFR) in Japanese men. We studied the cross-sectional association of BMI with eGFR in 3872 Japanese men in a work-site population (18-64 y; mean age 42.1 ± 0.2 y). Estimated glomerular filtration rate was calculated by a novel equation for Japanese men. Estimated glomerular filtration rate was negatively correlated with age, systolic blood pressure (SBP), hemoglobin A1c (HbA1c), and BMI. We performed multiple regression analysis, controlling for factors, such as SBP, low-density lipoprotein-cholesterol, gamma-glutamyl transpeptidase, age, HbA1c, and uric acid. The association between age and eGFR was highly statistically significant. In addition, BMI was still significantly associated with eGFR independently of SBP. Moreover, mean eGFR, which was adjusted for age, SBP, HbA1c, serum uric acid, and gamma-glutamyl transpeptidase, decreased from 88.9 mL/min/1.73 m(2) in the first quartile of BMI to 87.5 mL/min/1.73 m(2) in the second, 86.9 mL/min/1.73 m(2) in the third, and 85.9 mL/min/1.73 m(2) in the fourth quartile (test for trend, P < .0001). These results show that a close relationship is present between obesity and decreased eGFR in Japanese men. Keeping appropriate body weight, in addition to appropriate blood pressure, in young and middle age may be important to prevent renal damage in older age.