Background: Fibrous cap thickness (FCT) is one of the key features of coronary vulnerable plaque. FCT is measured at an arbitrary point, determined on visual assessment of 2-D cross-sectional imaging. This method has poor reproducibility. The aim of this study was to compare the 3-D structure of FC in non-culprit lipid plaques between patients with ST-elevation myocardial infarction (STEMI) and with stable angina (SA) on optical coherence tomography. Methods and Results: A total of 54 non-culprit plaques from 23 STEMI and 23 SA patients were evaluated. Thin cap fibroatheroma (TCFA), defined as lipid plaque with FCT <80 µm, was identified using a novel algorithm. The number of TCFA, surface area of each TCFA, and the sum total area of TCFA in the target vessel were measured. Patients with STEMI had a greater median number of TCFA (9, IQR 1–17 vs. 2, IQR 0–5; P=0.002), the largest median single TCFA area (0.40, IQR 0.14–0.69 vs. 0.08, IQR 0.04–0.16 mm2; P<0.001) and median sum total area of TCFA (1.04, IQR 0.41–1.95 vs. 0.24, IQR 0.08–0.48 mm2, P<0.004). Conclusions: Patients with STEMI, as compared with those with SA, have greater vulnerability to non-culprit plaque.