The differential diagnosis of fasting hyperglycemia in type I diabetes includes the Somogyi effect, the dawn phenomenon, and insufficient insulin administration. To determine the causes of fasting hyperglycemia and their effect on subsequent daytime blood glucose control, the authors retrospectively reviewed blood glucose profiles of 126 patients with type I diabetes. The Somogyi effect accounted for 12.6% of all instances of fasting hyperglycemia, the dawn phenomenon, 24.1%, and poor control, 63.3%. Measurement of 3 AM and 5 AM blood glucose values is the key to making a correct diagnosis. Once a patient's fasting hyperglycemia is placed in one of these groups, appropriate treatment can be started.