Aims: The SYNTAX-LE MANS substudy prospectively evaluated 15-month angiographic and clinical outcomes in patients with treated left main (LM) disease. Methods and results: In the SYNTAX trial, 1,800 patients with three-vessel and/or LM disease were randomised to either CABG or PCI; of these, 271 LM patients were prospectively assigned to receive a 15-month angiogram. The primary endpoint for the CABG arm was the ratio of >= 50% to <100% obstructed/occluded grafts bypassing LM lesions to the number placed. The primary endpoint for the PCI arm was the proportion of patients with <= 50% diameter stenosis ('patent' stents) of treated LM lesions. Per protocol, no formal comparison between CABG and PCI arms was intended based on the differing primary endpoints. Available 15-month angiograms were analysed for 114 CABG and 149 PCI patients. At 15 months, 9.9% (26/263) of CA BG grafts were 100% occluded and an additional 5.7% (15/263) were >= 50% to <100% occluded. Overall, 27.2% (31/114) of patients had 21 obstructed/occluded graft. The 15-month CABG MACCE rate was 8.8% (10/114) and MACCE at 15 months was not significantly associated with graft obstruction/occlusion (p=0.85). In the PCI arm, 92.4% (134/145) of patients had <= 50% diameter LM stenosis at 15 months (89.7% [87/97] distal LM lesions and 97.9% [47/48] non-distal LM lesions). The 15-month PCI MACCE rate was 12.8% (20/156) and this was significantly associated with lack of stent patency at 15 months (p<0.001), mainly due to repeat revascularisation. Conclusions: At 15 months, 15.6% (41/263) of grafts were at least 50% obstructed but this was not significantly associated with MACCE; 92.4% (134/145) of patients had stents that remained patent at 15 months, and stent restenosis was significantly associated with MACCE, predominantly due to revascularisation.