Glucose-dependent insulinotropic polypeptide (GIP) secretion in diabetic Europeans with type 1 (T1DM) and type 2 (T2DM) following test meal (TM) has been shown to be normal. In Japanese patients with T2DM, GIP secretion was also normal. We determined whether GIP secretin is influenced by various factors. Plasma glucose (PG), serum insulin (s-IRI), serum C-peptide (s-CPR), and plasma total GIP (p-total GIP) levels were measured at 0, 30, and 60 minutes after TM (560 kcal) in patients with T1DM (n = 15, group 1) and T2DM (n = 29, group 2) treated with various medications. HbA1c was also measured. At baseline, means of age, BMI, HbA1c, PG, s-CPR, SUIT (secretory unit in transplantation) and p-total GIP were significantly lower in group 1 than in group 2. Each mean of postprandial p-total GIP levels after TM in all patients was more dramatically increased than other factors. The area under the curve (AUC) of p-total GIP levels in early-phase (0 to 30 min) was significantly positively correlated with BMI in group 2 but not in group 1, and not with other factors. These results indicate that the GIP secretion after TM in diabetic Japanese patients was dramatically increased, and the AUC of GIP secretion in early-phase was positively correlated with BMI in non-obese and obese patients with T2DM, but not with T1DM. The increase was not influenced by gender, age, glycemic control, duration of disease, micro- or macro-vascular disturbances, or oral drugs.