Objective Diabetes develops as a bihormonal disease. Antagonizing glucagon action represents a new avenue for intervention of diabetes. This study investigated the serum glucagon levels in Korean patients with carbohydrate metabolism abnormalities. Methods Subjects who did not have carbohydrate metabolism abnormalities before were examined: normal subjects (CONTROL), impaired fasting glucose (IFG), impaired glucose tolerance (IGT), IFG/IGT, and type 2 diabetes of isolated fasting hyperglycemia (IFH), isolated postprandial hyperglycemia (IPH), and fasting and postprandial hyperglycemia. Plasma levels of glucose, insulin and glucagon were measured in the fasting state (GTT0), and at 30 (GTT30) and 120 (GTT120) minutes after glucose loading. Results Patterns of insulin response in IFG and IFH were similar to CONTROL. In other groups, insulin response showed the delayed hyperinsulinemia. In patients of diabetes with increased glycated hemoglobin of more than 7.5%, even though they were newly diagnosed, insulin secretion was markedly decreased. In CONTROL, there was no change of glucagon levels before and after glucose loading. In all patients, fasting glucagon levels were higher than CONTROL except IPH. In all patients, a rise of glucagon levels was observed at GTT30. There was impairment of glucagon suppression at GTT120 except IFG and IGT. The ratios of glucagon to insulin except IGT were higher than CONTROL at GTT30, while those at GTT120 were various according to subgroups. Conclusion According to our data, glucagon imbalance starts before diabetes is diagnosed. Further studies about different mechanisms affecting glucagon imbalance in various types of glucose intolerance are needed.