The purpose of this study was to determine whether the elimination or the alleviation of hyperglycemia would improve coronary flow velocity reserve (CFVR) using transthoracic Doppler echocardiography (TTDE). CFVR was measured by TTDE in the left anterior descending coronary artery in 49 poorly controlled diabetic patients before and after antidiabetic treatment and 15 well controlled diabetic patients also underwent the same measurements. The fasting blood glucose level in the poorly controlled patients reduced from 270 +/-106 mg/dl to 116+/-39 mg/dl at 20+/-15 days after the intensive treatment. Although baseline coronary flow velocity (CFV) did not change between the 2 measurements (19.9+/-6.9 cm/s vs 19.0+/-5.4 cm/s, p=NS), the hyperemic CFV increased significantly after the treatment (47.3+/-13.4 cm/s vs 55.4+/-13.2 cm/s, p<0.001). Thus, the CFVR improved significantly after the treatment (2.47+/-0.55 vs 2.98+/-0.56, p<0.001). Although there was minimal improvement in the control group (2.37+/-0.38 vs 2.50+/-0.37, p<0.05), the improvement in CFVR was significantly greater in the poorly controlled patients with intensive treatment (0.51+/-0.33 vs 0.12+/-0.19, p<0.001) than that in the control group. These results suggest that optimal hypoglycemic therapy is important to improve the CFVR in poorly controlled diabetic patients.