In this paper, we present a case of a female patient with clinically unstable angina pectoris and a bordeline stenosis in the proximal segment of the left anterior descending coronary artery as assessed by coronary angiography and intravascular ultrasound. Virtual histology revealed morphological criteria of a vulnerable plaque forming the culprit lesion. Optical coherence tomography visualized both fibrous cap fracture and a significant stenosis of a coronary artery caused by soft structures identified as mural thrombus covering the plaque surface. The image of atherosclerotic plaque obtained by optical coherence tomography enabled explanation of the cause of coronary instability and influenced subsequent management. The presented case illustrates usefulness of optical coherence tomography as an imaging method complementary to virtual histology and intravascular ultrasound in the diagnostic evaluation of selected patients with acute coronary syndromes. Application of optical coherence tomography in the assessment of vulnerable atherosclerotic plaques is discussed as related to the presented case.