Intravascular ultrasound imaging can detect intimal thickening and is suitable for detection of early atherosclerosis, which cannot be detected by conventional angiography. The aim of the present study was to investigate the epicardial coronary morphology and intracoronary pressure in relation to slow coronary flow (SCF). The study population consisted of 19 patients with SCF [11 (57.9%) females; 55.95 ± 9.42 years]. Proximal, middle, distal and mean total vessel area, lumen area, intima + media area (IMA), percent IMA, and maximal intima + media (I + M) thickness were calculated and compared to healthy subjects. Proximal, middle, distal and mean I + M thickness, IMA, and % IMA of patients with SCF were found to be significantly higher than those of control subjects. Longitudinally extended massive calcification throughout the epicardial arteries was found in 13 (68.4%) patients with SCF and regional calcification was found in 6 (31.6%) patients with SCF. Proximal and distal pressure gradients of patients with SCF were determined to be 15.84 ± 12.11 mmHg in the intracoronary pressure measurements. Fractional flow reserve values were significantly lower than the normal population (0.83 ± 0.13, P < 0.0001). This study indicates that patients with SCF have diffuse intimal thickening, widespread calcification along the vessel wall and atheroma which does not cause luminal irregularities in coronary angiography, and a pressure gradient between proximal and distal segments of epicardial coronary arteries with SCF. Based on these results, we believe that SCF may be a form of diffuse atherosclerosis involving both the microvascular system and epicardial coronary arteries.