Thin fibrous cap and lipid pool are thought to be major determinants of plaque instability. However, current imaging modalities such as angiography, intravascular ultrasound, and angioscopy do not have sufficient resolution to accurately measure them. It is widely accepted that optical coherence tomography (OCT) is the in vivo “gold standard” imaging modality for the measurement of fibrous cap thickness. However, its reproducibility has never been systematically studied. Therefore, we attempted a systematic investigation of interobserver agreement and intraobserver reproducibility of fibrous cap thickness and lipid arc measurements. Fifty frames and 25 pullback runs of OCT were randomly selected for frame ...