It is reported on the clinical evidence of the determination of creatinine and of the endogenic creatinine clearance. The determination of the serum creatinine gives a good parameter for the observation of the course in advanced renal insufficiency. The endogenic creatinine clearance which is often carried out in the clinical practice in the normal and moderately limited region allows in most cases sufficient measurement of the filtrate of the glomerulum. On 118 in-patients a method was tested which allows a nomographic estimation of the endogenic creatinine clearance on the basis of the actual serum creatinine level, taking into consideration age, sex, and bodyweight.