The early diagnosis of renal failure is not likely to be made often because the clinical signs and the commonly used laboratory procedures have been of little value unless renal failure was well established.1 The need for a reliable, quick, and easy method to predict or detect impairment of renal function early in its course is obvious. Early detection would permit the study of the causative factors implicated in the development of renal failure in the individual patient in time to be of benefit to him. It has been calculated that a sampling of 30,000 patients is needed to establish a significant statistical proof of the efficacy of a therapeutic agent upon renal failure.2 Data will be presented to show the value of frequent determinations of urinary urea concentration to detect and predict impairment of renal function after cardiac surgery and cardiopulmonary bypass. An equation has been developed