Background Using intravascular ultrasound (IVUS), we studied the impact of (1) pre-procedural IVUS findings on stent expansion and (2) pre- and post-procedural findings on long-term clinical outcomes in patients undergoing drug-eluting stent implantation for unprotected left main (LM) bifurcation disease. Methods Using left anterior descending artery (LAD) pullback, we evaluated ostial LAD (3mm distal to the carina), polygon of confluence (POC, confluent zone of LAD and left circumflex artery), and DLM (distal LM, 3mm just proximal to the POC). Measurements included minimum lumen area (MLA) and minimum stent area (MSA) within each segment. Results In 168 LM bifurcations, the pre-procedural MLA and the post-stenting MSA within the LM were located within the POC in 41% and 70%, respectively. Independent predictors for post-stenting MSA within the distal portion of LM above the LAD carina were pre-procedural lumen area of the LAD carina (棺=0.253, 95% CI=0.10-0.36, p=0.001) and pre-procedural MLA within the POC (棺=0.205, 95% CI=0.04-0.23, p=0.008). During the follow-up period of 41.8짹18.0 months, there were a total of 26 patients with cardiac events. On multivariable Cox model, female gender (adjusted HR=2.56, 95% CI=1.173-5.594, p=0.018) and pre-procedural MLA within the POC (adjusted HR=0.829, 95% CI=0.708-0.971, p=0.020) were the independent predictors for the occurrence of events at 3-year follow-up. Conclusion Assessed by LAD pullback, the pre-procedural MLA within the POC was a surrogate reflecting the overall severity of LM bifurcation disease, contributed to the post-stenting MSA within the distal segment of LM bifurcation, and was a predictor of clinical events during follow-up.