Background: Restenosis rates after percutaneous coronary intervention on bifurcation lesions are still high even with drug-eluting stents. However, the mechanism of restenosis, especially at the ostium of the side branch (SB), has not been fully understood. Methods and Results: We studied 73 bifurcation lesions, which were treated with 2 drug-eluting stents by provisional T stenting and small protrusion technique. Intravascular ultrasound (IVUS) studies were performed in both main vessels (MV) and SB immediately after procedure and at 9-month angiographic follow-up. IVUS analysis included 5 distinct locations: MV proximal, MV middle, MV distal, SB ostium, and SB distal. Post-procedural minimum stent area (MSA) was smaller and stent expansion was less significantly in the SB than the MV (P<0.001 and P=0.007, respectively). While post-procedural MSA was similar between the SB ostium and distal stent, MLA at the SB ostium had a tendency to be smaller than SB distal at follow-up (P=0.06). MLA at follow-up was significantly correlated with post-procedural MSA in the SB (Figure 1). Percentage of neointimal hyperplasia is significantly higher in the SB ostium compared with the MV proximal, MV distal, and SB distal stent (Figure 2). Conclusions: In bifurcation lesions treated with 2 stents, follow-up MLA correlated with post-procedural MSA significantly in the SB and neointimal hyperplasia is increased at the SB ostium. Our data suggest that obtaining adequate post-procedural MSA is crucial in avoiding restenosis in the SB ostium.