Background Optical coherence tomography (OCT) is a new imaging method. With a resolution of approximately 10-20 Ém, which is approximately 10-fold higher than that of intravascular ultrasound (IVUS). Methods and Results This study compared the coronary intima - media thickness (IMT) and the intimal thickness of 54 coronary arterial segments evaluated by histological examination with the results of OCT and IVUS. There was better agreement in IMT between OCT and histological examination than between IVUS and histological examination (r=0.95, p<0.001, mean difference =-0.01±0.07 mm for OCT; r=0.88, p<0.001, mean difference =-0.03±0.10 mm for IVUS). There was good agreement in the intimal thickness between OCT and histological examination (r=0.98, p<0.001, mean difference =0.01±0.04 mm). Conclusions IMT could be measured more accurately by OCT than IVUS. In addition, the intimal thickness could be evaluated by OCT and correlated well with the histological examination. (Circ J 2005; 69: 903 - 907)