Unstable angina pectoris (UAP) is categorized with the Braunwald classification. However, the association of clinical presentation and plaque structure/function has not yet been elucidated in relation to cause. We used optical coherence tomography to investigate this relation. One hundred fifteen patients with primary UAP were categorized according to the Braunwald classification. Patients with class I UAP had the highest frequency of ulcers without fibrous cap disruption (p = 0.003) and the smallest minimum lumen area (class I, median 0.70 mm(2), quartiles 1 to 3 0.42 to 1.00; class II, 1.80 mm(2), 1.50 to 2.50; class III, 2.31 mm(2), 1.21 to 3.00; p <0.001). Patients with class II UAP had the highest frequency of coronary spasm (p <0.001) and the lowest frequency of thrombi (p <0.001). Patients with class III UAP had the highest frequency of plaque ruptures (p <0.001), the thinnest fibrous cap (class I, median 140 microm, quartile 1 to 3 90 to 160; class II, 150 microm, 120 to 160; class III, 60 microm, 40 to 105; p <0.001), and the highest frequency of thin cap fibroatheromas (p <0.001) and spotty calcifications (p <0.001). In conclusion, the structures/functions of culprit lesions on optical coherence tomograms differ in the Braunwald classes of UAP. Plaque vulnerability, progressive stenosis, and vasoconstriction may be related to the cause of the distinct presentations.