Urinary β-glucuronidase activity was measured in dialyzed urine samples from normal subjects and patients with renal disease of diverse causes. The normal mean value was 14.4 enzyme units with a standard deviation of 7.1 and a range of 3.1 to 32.0 enzyme units. Urinary enzyme activity was increased in the majority of patients with active renal parenchymal disease, particularly those diseases resulting in tubular injury. Measurement of urinary β-glucuronidase activity was useful in detecting acute tubular necrosis, early renal transplant rejection, active pyelonephritis, and renal adenocarcinoma. It was not possible to differentiate upper from lower urinary tract infection.