Spontaneous diabetes mellitus has been characterized in a line of nonobese purebred keeshond dogs as an insulin-requiring hereditary disorder with onset at between 2 and 6 mo of age. Diabetic dogs developed cataracts, became ketotic, hyperglycemie, hypercho-lesterolemic, lipemic, and hypoinsulinemic. Basal glucagon, cortisol, and T4 serum concentrations and responses to ACTH, TSH, and arginine were normal. Light microscopic studies of the pancreas by immunocytochemical procedures revealed the absence of islet B cells, the presence of A cells, and solitary B cells. Diabetic dogs had poor fecundity, and a single puberal diabetic male had poor semen quality and was unable to sire pups. Parents of diabetics and nondiabetic siblings were normal. This spontaneous form of diabetes mellitus, with similar lesions to the insulin-dependent diabetes of people, will be a valuable aid to comparative biomedicai research of diabetes mellitus.