Objective: We hypothesized that patients presenting with stent thrombosis (ST) have a high prevalence of stent underexpansion and malapposition when assessed by intravascular ultrasound (IVUS). Background: IVUS can provide mechanistic insight into mechanical factors, including stent underexpansion, malapposition, and fracture that may predispose to ST. Methods: All consecutive cases of angiographically confirmed ST from a multicenter registry (from 2005 to 2010) were reviewed. All IVUS images were reviewed off-line for the presence of stent underexpansion, malapposition, and fracture. Kaplan–Meier analysis was used to determine whether use of IVUS at the time of ST was associated with long-term mortality and major adverse cardiovascular events. Results: IVUS was performed in 32 of 173 subjects with ST (18%). Stent underexpansion was present in 82% of cases and in all cases of early ST, with a mean stent expansion of 0.7 ± 0.23 by MUSIC criteria. Stent malapposition was most frequently observed in very late ST (40%). In-hospital mortality was similar between subjects who had IVUS performed at the time of ST when compared with the non-IVUS group (3.2% vs. 4.3%, P = 0.8). Subjects who had IVUS performed at the time of ST had lower rates of mortality (HR 0.4, 95% CI 0.1-1.6, P =0.2) and major adverse cardiovascular events (HR 0.5, 95% CI 0.2–1.4, P =0.2) at follow-up, but these values were not statistically significant. Conclusions: There is a high prevalence of stent underexpansion in early ST, while the prevalence of malapposition is higher in very late ST. Use of IVUS during treatment for ST may identify mechanisms underlying the development of ST. © 2012 Wiley Periodicals, Inc.