A formula has been developed to predict creatinine clearance (Ccr) from serum creatinine (Scr) in adult males: Ccr = (140 – age) (wt kg)/72 × Scr(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 Ccr were predicted by this formula and four other methods and the results compared with the means of two 24-hour Ccr’s 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.