Objective Our objective was to define the most appropriate treatment for acute coronary syndrome (ACS) in patients with malignancy. Methods and Results The BleeMACS project is a worldwide multicenter observational prospective registry in 16 hospitals enrolling patients with ACS undergoing percutaneous coronary intervention. Primary endpoints were death, re-infarction, and major adverse cardiac events (MACE; composite of death and re-infarction) after 1 year of follow-up. The secondary endpoint was bleeding events during follow-up. We performed sub-study analyses according to whether beta-blockers (BBs), angiotensin-converting enzyme inhibitors (ACEIs) or angiotensin receptor blockers (ARBs), statins, or proton pump inhibitors (PPIs) were prescribed at discharge. We also calculated the propensity score for optimal medical therapy (OMT; combination of BB, ACEI/ARB, and statins). The study included 926 patients. According to the multivariate analysis, ACEIs/ARBs (hazard ratio [HR] 0.58, 95 % confidence interval [CI] 0.36-1.94; p = 0.03) and statins (HR 0.37, 95 % CI 0.23-0.61; p < 0.01) reduced the risk of MACE, while the effects of BBs (HR 0.85, 95 % CI 0.55-1.32; p = 0.48) and PPIs (HR 1.33, 95 % CI 0.83-2.12; p = 0.23) were not significant. OMT was prescribed at discharge in 300 (32.4 %) patients; after propensity score analysis, OMT showed a significant reduction in death (3 % vs. 12.5 %, HR 0.21, 95 % CI 0.1-0.4; log-rank p < 0.001) and MACE (6.7 vs. 15.2 %, log-rank p = 0.01). Conclusion In patients with ACS and malignancy, OMT reduces the risk of adverse events at 1 year; in particular, ACEIs/ARBs and statins were the most protective drugs. .