Background: The polygon of confluence (POC) represents the zone of confluence of the distal left main (LM), ostial left anterior descending (LAD), and ostial left circumflex (LCX) arteries. Methods: We used intravascular ultrasound (IVUS) to assess the POC pre and post-drug-eluting stent implantation for unprotected distal LM disease. Four segments within 82 LM bifurcation lesions were defined by longitudinal IVUS reconstruction: (1) ostial LAD, (2) POC, and (3) distal LM (DLM)—from LAD-pullback, and (4) ostial LCX from LCX-pullback. Results: Preprocedural minimum lumen area (MLA) and poststenting minimum stent area (MSA) within the LM were mainly located within the POC (51 and 71%). On ROC analysis, a cut-off of the MLA within the POC of 6.1 mm2 predicted significant LCX carinal stenosis (85% sensitivity, 52% specificity, AUC = 0.7, 95% CI = 0.57–0.78, P < 0.01). Poststenting MSA within the distal LM proximal to the carina (to include DLM and POC) positively correlated with the preprocedural MLA within the POC (r = 0.283, P = 0.02); it was significantly smaller in 48 lesions with a pre-PCI MLA within the POC < 6.1 mm2 versus 25 lesions with a pre-PCI MLA ≥6.1 mm2 (7.5 ± 2.1 mm2 vs. 8.6 ± 2.0 mm2, P = 0.04). Independent predictors for poststenting LCX carinal MLA also included preprocedural MLA within the POC (β = 0.240, 95% CI = 0.004–0.353, P = 0.04). Conclusion: The MLA within the POC was a good surrogate reflecting the overall severity of LM bifurcation disease including ostial LCX stenosis pre-PCI and the ability to expand a stent within the distal LM as well as final ostial LCX lumen area post-PCI. © 2011 Wiley Periodicals, Inc.