Trehalase (an enzyme decomposing the disaccharide trehalose) activity was studied in 29 healthy subjects, 25 patients with cirrhosis and 112 diabetics. Mean trehalase activity was 176 +/- 11 units in the control group, 647 +/- 421 units in the patients with cirrhosis and 467 +/- 239 units in diabetics. The differences between the control group on the one hand and the groups with cirrhosis and diabetes on the other were statistically significant. The results show that the organism, under pathological conditions, makes far greater use of its enzymatic apparatus to assure its basic requirements, but the scatter of the values is so great that the determination of trehalase has no discriminative value in individual cases.