Coronary slow flow (CSF) is characterized by delayed opacification of coronary arteries in the absence epicardial occlusive disease. QT duration and dispersion have been reported to be longer in patients with CSF. ACE inhibitors may improve CSF through positive effects on endothelial function. The study included 32 patients having CSF and 25 subjects having normal coronary arteries in coronary angiography. The patients were evaluated with 12-leads electrocardiography and echocardiography before and 3 months after treatment with perindopril. Compared to the control group, maximum corrected QT duration (QTcmax) (432.0 ± 28.9 vs. 407.0 ± 39.1 ms, p = 0.008) and QT dispersion (QTcD) (64.0 ± 16.5 vs. 37.3 ± 12.1 ms, p < 0.001), mitral inflow deceleration time (DT) (251.3 ± 49.4 vs. 218.8 ± 44.5 ms, p = 0.013), and isovolumetric relaxation time (IVRT) (115.8 ± 18.4 vs. 107.2 ± 22.9 ms, p < 0.001) were significantly longer and E/A ratio 0.85 ± 0.2 vs. 1.1 ± 0.3, p = 0.004) was lower in patients with CSF. QTcmax (to 407.0 ± 28.0 ms, p = 0.001), QTcD (to 44.5 ± 11.4 ms, p < 0.001), DT (to 221.6 ± 37.7 ms, p < 0.001) and IVRT (to 103.8 ± 16.1 ms, p < 0.001) were significantly decreased and E/A ratio (to 0.98 ± 0.3, p < 0.001) was significantly increased after treatment with perindopril. Coronary slow may be associated with prolonged QT interval and increased QT dispersion and impaired diastolic filling. Perindopril may be helpful in restoration of these findings.