Insulin-induced hyperglycemia was diagnosed in 8 dogs with diabetes mellitus. Owners sought veterinary care because of polydipsia, polyuria, polyphagia, persistent morning glycosuria, or seizures in their dogs. These abnormalities had persisted despite increasing the dosage of insulin. Insulin-induced posthypoglycemic hyperglycemia was diagnosed by determining blood glucose concentrations every 2 hours during a 24-hour period, beginning at 8 A.M. Wide fluctuations in the blood glucose concentration were reduced by decreasing the daily insulin dose. The signs observed by the owners disappeared after the insulin dose was reduced.