In 45 diabetics the 24-h urinary excretion of β-N-acetylglucosaminidase (E.C. 3.2.1.30) was increased by 40% (P < 0.05) compared to 35 control subjects. The enzyme excretion was correlated with glycemia (r = 0.58, P < 0.001), being little changed in diabetics with blood glucose concentrations of less than 200 mg/dl, and markedly elevated (+ 123%, P < 0.001) in those whose blood glucose was greater than 200 mg/dl. The rate of diuresis seemed to have no effect. These data indicate that the enhanced activity previously described in sera of diabetics for β-N-acetyl-glucosaminidase (as well as for other lysosomal enzymes) cannot be attributed to impairment of renal excretion, and support the hypothesis that in diabetes there is an "activation" of lysosomal enzymes in tissues that causes an increase in their activity in serum and, consequently, in urine.