The incidence of thrombosis in patients with intracranial atherosclerotic stenosis (ICAS) remains unclear. Optical coherence tomography (OCT) has the potential to explore the vessel wall structure of posterior-circulation ICAS because of its relatively straight anatomical structure compared with that of the anterior cerebral arteries. This study aimed to determine the prevalence and characteristics of thrombosis in the posterior-circulation ICAS using OCT.This prospective study was conducted on 135 patients with posterior-circulation arterial stenosis who underwent OCT. All patients were symptomatic and had a severely stenotic lesion (70-99%) in the vetebrobasilar artery. The enrolled patients were classified according to the presence of in situ thrombus as defined by OCT. Clinical data and OCT characteristics were compared.Eighty-two patients diagnosed with posterior-circulation ICAS were enrolled. In situ thrombi were identified in 34 patients. Clinically, patients with in situ thrombus were more prone to cerebral infarctions than transient ischemic attacks. The percentage area of stenosis in the non-thrombus group was significantly lower than that in the thrombus group. The thrombus burden, mean flow area, mean thrombus area, maximum lipid arc, and mean lumen area were significantly different among white, red, and mixed thrombi.We achieved in vivo vessel wall structural analysis of posterior-circulation ICAS with the largest sample size. We also revealed the true incidence of in situ thrombosis and potential corresponding clinical events of posterior-circulation ICAS for the first time.© Author(s) (or their employer(s)) 2024. No commercial re-use. See rights and permissions. Published by BMJ.