Hypoglycemia in a dog due to an insulinoma was first reported in the literature in 1927. Although there have been many reported cases since that time, it is still considered to be a fairly rare condition by most authors. Its frequency, however, is probably much greater than the literature would lead one to believe, as the vast majority of cases go unreported or unrecognized. The Iowa State University Teaching Hospital alone diagnoses three or four cases each year, which is a substantial number in light of the fact that functional beta cell tumors have been reported in only 45 dogs.