In sera of diabetics (115 patients) the activity of the lysosomal enzyme betaglucuronidase was found increased by 63 per cent (p<0.001) compared to normal individuals (75 subjects). The enzyme level seemed correlated to both the degree of glucose metabolic disorder and the vascular lesions. In fact, among the patients without vascular complications, beta-glucuronidase activity was roughly parallel to glycemia (r=0.647, p<0.001), reaching very high values (about four times the normal) in six decompensated ketoacidotic patients, while in diabetics with essentially similar glycemia, the enzyme was higher (p<0.005) in subjects with vascular complications, either microangiopathic or macroangiopathic in nature.Since lysosomal enzymes are capable of degrading various molecules, the increase of serum beta-glucuronidase in diabetics was regarded as an index of involvement of lysosomes in tissues, leading to activation of lysosomal hydrolases, apparently in response to the metabolic need of degrading either compounds which have accumulated in tissues, such as mucopolysaccharides and glycoproteins in diabetics with vasculopathies, or various constituents of cells themselves in a context of increased tissue catabolism, as occurs in diabetics with severe metabolic disorder.