Qualitative and quantitative studies were made of the pancreas of a series of infants of diabetic and nondiabetic mothers. The total mass of islet tissue of infants of diabetic mothers was both absolutely and relatively increased. Histologically, the islets of infants of diabetic mothers were characterized by hypertrophy of islet cells, nuclear alterations of beta cells, and prominent vascularity of islets. In some patients there was also a pronounced eosinophilic leukocytic infiltration of interstitial tissues accompanied by crystalline masses and f ibrosis. The presence in some infants of diabetic mothers of a subacute interstitial pancreatitis suggests that factors other than a physiologic adaptation to maternal hyperglycemia may play a role in the pathogenesis of these pancreatic lesions.