In this study, we described the effectiveness of metformin on Japanese type 2 diabetes patients receiving sulfonylureas and the clinical characteristics of the patients whose glycemic control were significantly improved with metformin administration. Our results showed that the reduction of glycohemoglobin (HbA1C), serum concentration of total cholesterol, and diastolic blood pressure was statistically significant through the administration of metformin. The clinical characteristics of the patients who responded to metformin therapy exhibited lower systolic blood pressure in addition to higher HbA1C value just before administration of metformin when compared with DeltaHbA1C (HbA1C 6 months after administration of metformin--HbA1C before administration of metformin). Moreover, effectiveness of metformin was weakened, in comparison with non-hypertensive patients, even though the blood pressure of hypertensive patients was reduced to normal range by medication with antihypertensive drugs. But average reduction of HbA1C level of hypertensive patients without antihypertensive medications was smaller than those of patients with high blood pressure with such medication. These results suggested that high blood pressure and hypertension phenotype itself were suppressive factors of metformin but antihypertensive therapy itself enhanced the effectiveness of metformin regardless of the improvement of blood pressure.