We studied the efficacy of sitagliptin in type 2 diabetic patients of our outpatient clinics. Since December in 2009, 164 patients have been treated by sitagliptin for their management of diabetes. HbA1c decreased by 0.8% in all patients without any change in mean body weight after 3 months. However, actually HbA1c did not decrease in 30 patients, and more than half of patients showed weight gain to some extent. Patients were classified according to the reduction of HbA1c and analyzed based on this category. Baseline characters such as age, gender, duration of diabetes, BMI, concomitantly used oral hypoglycemic agents and the score for life-style assessment were not related to glucose-lowering effect of sitagliptin. Ninety eight patients whose HbA1c had decreased after 3 months were further followed-up for another 3 months. Among them 45 patients showed some relapsing of HbA1c after 6 months, and they were compared with 53 patients without relapsing. More cases had been switched from α-glucosidase inhibitor (α-GI) and the score for life-style assessment was lower in relapsing patients compared to those in patients without relapsing. In conclusion, clinicians should keep the fact in mind that the individual variation of glucose-lowering effects and the possibility of relapsing exist during sitagliptin treatment, and that concern about life-style is still a quite important issue to prevent weight gain and the relapsing of blood glucose control.