Serum creatinine concentrations are widely used clinically as an index of renal function. In stable normal or reduced renal function they are determined by the rate of creatinine production and the endogenous creatinine clearance, and, during changing renal function, also by the apparent volume of distribution of creatinine. These determinants of serum creatinine concentrations, however, are affected by age, sex and body weight. The rate of creatinine production is proportional to body weight, and it decreases with age and is slower in females than in males. The endogenous creatinine clearance decreases with age and is lower in females than in males. The apparent volume of distribution of creatinine is equal to the total body water, which is proportional to body weight, and it decreases with age and is lower in females than in males. The individual relationships between the determinants of serum creatinine concentrations and age, sex, and body weight on the relationship between normal and reduced renal function and serum creatinine concentrations are illustrated by simulations. Equations are derived to predict endogenous creatinine clearance from serum creatinine concentrations, and a nomogram is presented for determining relative renal function from serum creatinine concentrations, which take into account the age, sex and body weight of the individual patient. It is recommended for rapid clinical evaluation of stable normal or reduced renal function. When rapid changes are expected clinically, however, it is recommended to use a mid urine collection period serum creatinine sample for creatinine clearance determination.