Three formulae which could predict creatinine clearance from serum creatinine values were derived from the data which were obtained from the study of 169 patients and healthy control subjects. Values for creatinine clearance which were obtained by using these three formulae and four other previous formulae were compared with the observed values. Most of the predicted and observed values differed by more than 20% of the observed values. When a formula which was derived by using the data from forty-five healthy control subjects was used to predict the creatinine clearance values of this group and the patients, there was a close agreement between the observed and the predicted values for only the healthy control subjects but not the patients. Because of the various factors which affect creatinine excretion in health and disease, a predictive formula which includes serum creatinine alone and not urinary creatinine might give unreliable values.