A body mass index (BMI)>or=25 kg/m2 increases the risk for long-term renal damage, possibly by renal hemodynamic factors. As epidemiological studies suggest interaction of BMI and sodium intake, we studied the combined effects of sodium intake and BMI on renal hemodynamics. Glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) were measured in 95 healthy men (median age 23 years (95% confidence interval: 22-24), BMI: 23.0+/-2.5 kg/m2) on low (50 mmol Na+, LS) and high (200 mmol Na+, HS) sodium intake. Mean GFR and ERPF significantly increased by the change to HS (both P<0.001). During HS but not LS, GFR and filtration fraction (FF) positively correlated with BMI (R=0.32 and R=0.28, respectively, both P<0.01). Consequently, BMI correlated with the sodium-induced changes in GFR (R=0.30; P<0.01) and FF (R=0,23; P<0.05). The effects of HS on GFR and FF were significantly different for BMI>or=25 versus <25 kg/m2, namely 7.8+/-12.3 versus 16.1+/-13.1 ml/min (P<0.05) and -0.1+/-2.2 and 1.1+/-2.3% (P<0.05). FF was significantly higher in BMI>or=25 versus <25 kg/m2, (22.6+/-2.9 versus 24.6+/-2.4%, P<0.05) only during HS. ERPF was not related to BMI. Urinary albumin excretion was increased by HS from 6.0 (5.4-6.7) to 7.6 (6.9-8.9). Results were essentially similar after excluding the only two subjects with BMI>30 kg/m2. BMI is a determinant of the renal hemodynamic response to HS in healthy men, and of GFR and FF during HS, but not during LS. Consequently, HS elicited a hyperfiltration pattern in subjects with a BMI>or=25 kg/m2 that was absent during LS. Future studies should elucidate whether LS or diuretics can ameliorate the long-term renal risks of weight excess.