1. Anatomical lesions of the pancreas occur in more than seven-eighths of all cases of diabetes mellitus. 2. In diabetes associated with lesions of the pancreas, the islands of Langerhans constantly show pathological changes (sclerosis, hyaline degeneration, infiltration with leucocytes and hypertrophy). 3. In some cases of pancreatic diabetes (twelve of ninety cases) the lesion of the pancreas is limited to the islands of Langerhans. 4. In sixteen cases of diabetes associated with hyaline degeneration of the islands of Langerhans the average duration of the disease has been three and a half years; in forty-six cases with sclerosis of these bodies, three years and eleven months. In six cases of diabetes associated with an infiltration of leucocytes about the islands of Langerhans the average duration has been eleven months. 5. Destructive lesions of the islands of Langerhans may be associated with compensatory hypertrophy of other interacinar islands. 6. Peculiar adenoma-like hypertrophy of the islands of Langerhans occurs in a small proportion of cases (seven of ninety) and may be associated with adenomata of the thyroid gland (two cases) and of the pituitary body (one case). 7. Diabetes mellitus occurring in association with hæmochromatosis (bronzed diabetes) is referable to pigmentation and destruction of the islands of Langerhans. 8. The pancreas is found to exhibit no pathological changes in twelve per cent. of cases. In approximately one-half of these cases it has been noted that the size of the gland or the number of islands is much less than normal. 9. Fifty per cent. of cases of diabetes mellitus occurring before the age of thirty years are associated with lesions of the pancreas; seventy-five per cent. of all cases of diabetes in which the pancreas is normal occur before the age of thirty years. Ninety-seven per cent. of cases of diabetes occurring after the age of thirty years are associated with lesions of the pancreas; and eighty-six per cent. occur in association with chronic interacinar pancreatitis accompanying arteriosclerosis. 10. Interacinar pancreatitis which occurs in seventy-three per cent. of all cases of diabetes is almost constantly associated with arteriosclerosis: gangrene of the extremities, which occurs with one-fourth of all cases of interacinar pancreatitis, is doubtless referable to the same cause. 11. Chronic interlobular pancreatitis, when associated with diabetes, is accompanied by sclerosis or hyaline degeneration of the islands of Langerhans. 12. Diabetes in association with myxœdema or with exophthalmic goiter may be referable to a lesion of the pancreas, namely, chronic interacinar inflammation with sclerosis of the islands of Langerhans; diabetes in association with acromegaly may be referable to a lesion of the islands of Langerhans, namely, sclerosis and hyaline degeneration with adenoma-like hypertrophy.