Lactic acid dehydrogenase (LDH) and one or more substances that inhibit its activity are constantly present in human urine. Dialysis removes the inhibitor or inhibitors and makes it possible to define the normal range of LDH activities (550 to 2,050 units per 8-hr volume of urine determined at 25 C [77 F]). Small but significant elevations may therefore be detected. Simultaneous determinations of LDH activity in urine and serum, of blood urea nitrogen, of total urinary protein, and of other urinary constituents were carried out on 50 patients. Increased LDH activities in the serum were not accompanied by commensurate increases in the urine. LDH activities in the urine had no detectable relation to the concentrations of other urinary constituents and were not affected by the presence of hematuria. High LDH activities in the urine appeared to be related only to disease of the urinary tract, particularly renal and vesical carcinoma.