Summary—Fifty‐five patients with chronic urinary retention and incipient or actual renal failure were studied. In the majority of patients renal function improved after bladder decompression, irrespective of whether or not a diuresis occurred. Excessive loss of salt and water was rarely a matter of concern and most patients did not require intravenous fluid replacement. Several lost weight and experienced a fall in blood pressure during the period of diuresis without adverse effect upon renal functional recovery. A profound fall in blood pressure occurred in only three patients, all of whom required long‐term sodium supplementation. It is concluded that the problem of salt and water loss after bladder decompression in patients with renal failure is exaggerated and difficult to predict. Over‐enthusiastic replacement of fluid in strict accordance with output could readily lead to fluid overload and prolongation of the diuretic period. Therefore fluid replacement should be determined by the clinical condition of the patient and measurement of improving renal function with less emphasis on urine output and its electrolyte