The impact of age, sex, race, and body build on kidney weight was examined in 357 normal individuals who died suddenly. Mulivariate, stepwise linear-regression analysis demonstrated that kidney weight was best predicted by body surface area. Age, sex, and race had no additional impact on kidney weight once differences in body build were taken into account. Similar results were obtained with heart and liver weights. These findings were confirmed using a second, independent paired sample of 16 lean and 16 massively obese adults. Body build-associated changes in kidney weight were paralleled by changes in both glomerular and tubulointerstitial areas. Body build is the major determinant of kidney weight and nephron size in normal humans.