The body mass index (weight in kilograms/height in square meters) is a common surrogate for fatness. With the advent of bioelectrical impedance analysis, more precise measurement of fatness in populations is now possible. We measured height, weight, and percentage that is fat by bioelectrical impedance analysis in 2032 adults, ages 31 to 92, participating in the Framingham studies. Body mass index was a poor predictor of fatness in women (R2 = 0.55) and men (R2 = 0.38), and was imprecise (standard error of estimate = 5 percentage points). The relationship between percentage fat and body mass index was quadratic in both sexes, and was altered by age in women (P < .0001) and, to a lesser extent, in men (P < .027). These data suggest that body mass index is an imprecise measurement of fatness compared with bioelectrical impedance.