From a morphological point of view, two distinct pathological pictures emerge in the common types of human diabtetes. The first is characterized in the initial phase of the disease by inflammatory lesions in the islets of Langerhans. These lead to fibrosis and apparent atrophy of the islets, which in a more advanced stage are composed of non-insulin-producing cells, the B-cells having been largely destroyed. This picture is specific and diagnostic for the classical insulin-dependent type of diabetes, which occurs more often in young individuals. The second is characterized by a variable, but on the average moderate reduction of the number of B-cells, and by degenerative islet changes (fibrosis, hyalinosis). Despite the long lasting hyperglycemia to which they have been submitted, the B-cells fail to develop cytological features of hyperactivity. This pathological picture is associated with the classical insulin-independent, maturity-onset type of diabetes.