A formula has been developed to predict creatinine clearance (C<sub>cr</sub>) from serum creatinine (S<sub>cr</sub>) in adult males: Ccr = (140 – age) (wt kg)/72 × S<sub>cr</sub>(mg/100ml) (15% less in females). Derivation included the relationship found between age and 24-hour creatinine excretion/kg in 249 patients aged 18–92. Values for C<sub>cr</sub> were predicted by this formula and four other methods and the results compared with the means of two 24-hour C<sub>cr’s</sub> measured in 236 patients. The above formula gave a correlation coefficient between predicted and mean measured Ccr·s of 0.83; on average, the difference between predicted and mean measured values was no greater than that between paired clearances. Factors for age and body weight must be included for reasonable prediction.