Background and Objectives: Stent underexpansion (SUE) is still known as the main factor of stent thrombosis and restenosis in the era of drug-eluting stent (DES). The aim of this study is to evaluate clinical, angiographic, and intravascular ultrasound (IVUS) determinants of SUE in DES. Subjects and Methods: The Post-stent Optimal Expansion (POET) trial is a prospective, multicenter, randomized trial aimed at demonstrating the efficacy of the non-compliant adjuvant balloon compared with semi-compliant balloon with CypherTM or TAXUSTM randomly allocated. All enrolled patients (n=301) were divided into 2 groups based on a final minimal stent cross-sectional area (CSA) <5 mm2, regarded as SUE [SUE group (n=33)], and ���5 mm2 [non-SUE (n=268)]. Then, we performed multivariate regression to find each determinant for SUE of the various clinical, angiographic, and IVUS parameters. Results: There were no differences in clinical variables between the 2 groups (SUE vs. non-SUE: mean age; 62짹9 vs. 60짹11 yrs, p=0.526, diabetes; 27% vs. 30%, p=0.450). The main quantitative coronary angiographic (QCA) determinants for SUE were preprocedural distal reference vessel (RV) diameter [odds ratio (OR)=0.08, 95% confidence interval (CI)=0.01-0.65, p<0.001] and postprocedural mean RV diameter [OR=0.004, 95% CI=0.001-0.02, p<0.001]. Analyzing preprocedural IVUS parameters, only 48 % of all cases analyzed, the main determining factor for SUE was pre-procedural distal RV external elastic membrane (EEM) CSA (OR=0.44, 95% CI=0.27-0.71, p=0.001). Comparing the incidences of SUE at the nominal pressure of stenting with those at the high pressure with post-dilation, SUE significantly decreased by the use of high pressure balloon [135 (45%) vs. 33 (11%), p<0.001]. Conclusions: There were no contributing clinical factors for SUE. The main determinants for SUE were preprocedural distal RV diameter and postprocedural mean RV diameter on QCA and distal RV EEM on IVUS. In addition, SUE occurred frequently at the nominal stent pressure without post-dilation with adjunctive balloon. Further data including IVUS analysis of all enrolled cases and the relationships with optimal expanded stent will be presented later.