RECENTLYA METHOD for the accurate measurement of the lactic dehydrogenase (LDH) activity of human urine has been developed.1By use of this method it has been shown that the urinary LDH activity is elevated in patients with carcinoma of the kidney and bladder and that it can be employed for the detection and diagnosis of these tumors.2,3 Measurement of urinary LDH activity in patients with other types of renal disease has not previously been performed. In the present study the urinary LDH activity has been measured in a large series of patients with renal disease, and in patients with benign essential hypertension, with the object of evaluating its diagnostic potential in these diseases. The results show that urinary LDH activity is elevated in most patients with significant renal disease, and that elevations of activity correlate well with the histological and clinical activity of the disease. Moreover, the