Background-There is a paucity of data on the use of optimal medical therapy (OMT) in patients with complex coronary artery disease undergoing revascularization with percutaneous coronary intervention or coronary artery bypass grafting (CABG) and its long-term prognostic significance. Methods and Results-The Synergy Between Percutaneous Coronary Intervention With TAXUS and Cardiac Surgery (SYNTAX) trial is a multicenter, randomized, clinical trial of patients (n=1800) with complex coronary disease randomized to revascularization with percutaneous coronary intervention or CABG. Detailed drug history was collected for all patients at discharge and at the 1-month, 6-month, 1-year, 3-year, and 5-year follow-ups. OMT was defined as the combination of at least 1 antiplatelet drug, statin, β-blocker, and angiotensin-converting enzyme inhibitor/angiotensin receptor blocker. Five-year clinical outcomes were stratified by OMT and non-OMT. OMT was underused in patients treated with coronary revascularization, especially CABG. OMT was an independent predictor of survival. OMT was associated with a significant reduction in mortality (hazard ratio, 0.64; 95% confidence interval, 0.48-0.85; P=0.002) and composite end point of death/myocardial infarction/stroke (hazard ratio, 0.73; 95% confidence interval, 0.58-0.92; P=0.007) at the 5-year follow-up. The treatment effect with OMT (36% relative reduction in mortality over 5 years) was greater than the treatment effect of revascularization strategy (26% relative reduction in mortality with CABG versus percutaneous coronary intervention over 5 years). On stratified analysis, all the components of OMT were important for reducing adverse outcomes regardless of revascularization strategy. Conclusions-The use of OMT remains low in patients with complex coronary disease requiring coronary intervention with percutaneous coronary intervention and even lower in patients treated with CABG. Lack of OMT is associated with adverse clinical outcomes. Targeted strategies to improve OMT use in postrevascularization patients are warranted. Clinical Trial Registration-URL: http://www.clinicaltrials.gov.