Two unrelated male infants presented with brittle insulin-dependent diabetes mellitus in the first days of life. Subsequently they each developed severe secretory diarrhea, with stool volumes of more than 100 ml/kg/day. Extensive biochemical and serological investigation failed to reveal the etiology of the diarrhea. The infants, cared for at different institutions, underwent therapeutic trials of various agents including loperamide, cholestyramine, prednisone, indomethacin, and somatostatin analogue, without response. Both infants succumbed to septicemia and malnutrition related to diarrhea and poor control of glycemia. At autopsy, both were found to have absence of islets of Langerhans in the pancreas, and diffuse dysplastic changes in small and large intestinal mucosae. In particular, the entire alimentary tract in each case was lined by epithelia most typical of foregut mucosa: secretory-type glands, absent crypts of Lieberkuhn, and absent villi. These cases are contrasted with previously-reported infants with congenital diabetes mellitus, and the possible interrelation of these two highly unusual findings, congenital diabetes mellitus and diffuse intestinal dysplasia, is examined.