In adults with diabetes mellitus, hepatomegaly and abnormalities of liver enzymes occur as a consequence of hepatocellular glycogen accumulation, as has been well described in children. During periods of hyperglycemia glucose freely enters the hepatocytes driving glycogen synthesis, which is augmented further by administration of insulin to supraphysiologic levels. The accumulation of excessive amounts of glycogen in the hepatocytes is a function of intermittent episodes of hyperglycemia and hypoglycemia and the use of excessive insulin. Hepatic glycogenosis occurs in patients with poorly controlled insulin-dependent type I or type II diabetes. The clinical manifestations of this phenomenon may include abdominal pain and obstructive symptoms such as early satiety, nausea, and vomiting. Ascites has rarely been reported. The typical biochemical findings are mildly to moderately elevated aminotransferases, with or without mild elevations of alkaline phosphatase. Liver synthetic function is usually normal. All these abnormalities, including the hepatomegaly, are readily reversible with sustained euglycemic control. The other major cause of hepatomegaly in patients with diabetes is steatosis. This is a function of the body habitus and state of insulin resistance rather than glycemic control. However, the distinction between steatosis and glycogenosis is important: whereas steatosis may progress to fibrosis and cirrhosis, glycogenosis does not, but reflects the need for better diabetic control. Glycogenosis and steatosis cannot be distinguished reliably on ultrasound examination. The histology, however, is definitive. In glycogenosis, as in primary glycogen storage diseases, there is excess glycogen in the cytoplasm, and often also in the nucleus, of hepatocytes. The hepatocytes throughout the lobule appear pale and swollen with clearly defined cell boundaries. Ultrastructural examination reveals cytoplasmic glycogen in clumps displacing organelles to the periphery of the cell, and there is little if any steatosis. We have shown that hepatomegaly due to glycogenosis in adults with diabetes is similar in all respects to the condition seen in children. As in children, liver enzyme abnormalities are unreliable in predicting the presence or the extent of glycogenosis. Hepatic glycogenosis can occur at any age, and therefore should be included in the differential diagnosis of hepatomegaly in all insulin-requiring diabetics.