BACKGROUND There are only a limited number of studies on the link between mild renal failure and coronary artery disease. The purpose of this study is to investigate the effects of mild renal failure on the distal vascular bed by measuring the coronary flow reserve (CFR) in transthoracic echocardiography after coronary artery bypass grafting (CABG). METHODS The study included 52 consecutive patients (12 women and 40 men) who had undergone uncomplicated CABG. The patients were divided into 2 groups. Group 1 included patients with a preoperative glomerular filtration rate (GFR) of 60-90 (mild renal failure), and group 2 included those with a GFR >90. The CFR measurements were carried out through a second harmonic transthoracic Doppler echocardiography. RESULTS The mean age was 60.08 ± 1.56 years in group 1 and 60.33 ± 1.19 in group 2. The mean preoperative CFR was 1.79 ± 0.06 in group 1 and 2.05 ± 0.09 in group 2. The mean postoperative CFR was 2.09 ± 0.08 in group 1 and 2.37 ± 0.06 in group 2. There was a statistically significant difference between the 2 groups as to preoperative creatinine clearance, preoperative estimated GFR, postoperative day 7 creatinine clearance, postoperative month 6 creatinine clearance, postoperative day 7 estimated GFR, postoperative month 6 estimated GFR, preoperative CFR, and postoperative CFR (P < .05). CFR was found to be unaffected by the choice of on-pump or off-pump technique (P = .907). After bypass surgery, there was a significant increase in the mean postoperative CFR, when compared with the mean preoperative CFR (P = .001). CONCLUSIONS In our study, we detected a decrease in CFR in patients with mild renal failure. We believe that in patients undergoing CABG for coronary artery disease, mild renal failure can produce adverse effects due to deterioration of the microvascular bed.