To investigate the cause of a low insulin secretory response to an oral glucose tolerance test (OGTT) in patients with anorexia nervosa (AN), we performed iv glucose tolerance tests (IVGTT) before and after treatment in 36 anorectic patients who showed low insulin secretion in response to the OGTT. These patients were subdivided into 3 groups by glucose tolerance curves during the OGTT: normal type, blood glucose level peaking 60 min or earlier after oral glucose ingestion; delayed type, blood glucose level peaking 90 min or later after oral glucose; and flat type, peak blood glucose level of 5.56 mmol/L or less after oral glucose. The results showed that the normal and flat type groups had normal glucose and insulin responses to iv glucose. In the delayed type group, in which the longest duration of AN before therapy was found, initial insulin secretion was decreased in response to both oral and iv glucose, indicating diminished pancreatic beta-cell function. After weight gain, this parameter improved significantly in both tests. The rate of glucose disappearance for the IVGTT was lower both before and after weight gain in this subgroup compared to that in normal controls, suggesting insulin resistance. In conclusion, the low insulin response to oral glucose seen in the flat type group may be due to the disturbance of gastrointestinal factors, such as motility. In contrast, the observations suggest that the delayed type group has beta-cell failure corrected by weight gain and has insulin resistance requiring a longer recovery time; these abnormalities are related to the duration of AN.