The study examined whether indexing glomerular filtration rate (GFR) for body surface area is appropriate for people who are severely overweight. Twenty normotensive adult men who were severely overweight but without microalbuminuria were enrolled into this study. The control group consisted of 20 healthy subjects matched for age, sex, and height. GFR was determined by measuring insulin with the continuous-infusion method. The clearance of endogenous creatinine was also measured after two daily urine collections. Renal plasma flow (RPF) was measured by p-aminohippurate clearance using the continuous-infusion method. Lean body weight was measured by impedentiometry. Adjusting for body surface area (in 1.73 m(2)) caused a significant reduction in GFR (P < 0.0001) in overweight humans (84.1 +/- 2.32 versus 109.6 +/- 3.07 mL/min/1.73 m(2)). The difference disappeared when GFR/height criteria were adopted. No difference between obese and healthy controls occurred after adjusting for lean body weight. Data for creatinine clearance paralleled those with insulin clearance; a significant reduction (P < 0.001) occurred after indexing for basal surface area, which disappeared after correction for height, as well as for lean body weight.