The use of the creatinine height index (CHI) as a measure of protein nutrition is reviewed. Any such cross-sectional measurement is inherently limited. Using published values for urinary creatinine excretion per kilogram body weight in adult subjects of varying age and values for "ideal" weight as a function of height, we have derived normal values for expected creatinine excretion in men and women of varying height. These permit the derivation of an age-corrected CHI. Possible explanations for the normal decrease in creatinine excretion with age include (1) decreasing lean body mass with age, (2) decreasing proportion of muscle in lean body, and (3) lower meat intake in older persons. Diet has an important influence if meat intake is substantial or if consumption of a creatine-free diet is prolonged. Creatinine metabolism and extrarenal excretion are minor, except in subjects with reduced renal function. Application of a correction for constant extrarenal clearance of creatinine in patients with chronic renal failure probably is not valid. Further observations of creatinine excretion in normal subjects of varying age and height are needed.