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Recently, coronary microvascular function was documented to be impaired even in patients with prehypertension. However, the impact of antihypertensive level on improvement of coronary microvascular dysfunction in hypertensive patients remains to be established. We investigated the optimal blood pressure achieved with treatment of antihypertensive agents for improvement of coronary microvascular dysfunction in hypertensive patients. We prospectively studied 108 untreated hypertensive patients. All patients were treated with antihypertensive agents for approximately 12 months. Coronary flow reserve (CFR) was measured before and after treatment. The patients were divided into hypertensive, prehypertensive, and normal groups based on their average blood pressure during the treatment period. Pretreatment CFR was similar among all groups. CFR increased significantly in the normal group during the treatment period, but decreased significantly in the prehypertensive and hypertensive groups. This decrease was significantly greater in the hypertensive group than in the prehypertensive group. Multivariate analysis showed lowering blood pressure to normal level to be an independent determinant of improvement in CFR. The normal group was divided into normal-higher and normal-lower subgroups based on their average diastolic blood pressure during the treatment period. Increase in CFR was significantly greater in the normal-higher group than in the normal-lower group. These findings indicate that lowering blood pressure to a normal level is necessary to improve coronary microvascular dysfunction in hypertensive patients. Furthermore, raising diastolic blood pressure to a higher level within normal range has the most beneficial effect. However, as this study is based on observational design, it may have several limitations.