 This is a randomized patients will be assigned to study treatment by chance, double-blind neither the patient nor the study doctor will know the identity of the assigned study treatment study to evaluate the safety and efficacy of rivaroxaban study drug compared to placebo a tablet identical in appearance to study drug but contains no active drug in patients with acute coronary syndrome ACS a condition where blood flow in a blood vessel in the heart is restricted because of a blood clot. Rivaroxaban is a drug that acts as a blood thinner and is being tested to see if it will be safe and effective in patients diagnosed ACS. The goal of this study is to identify the dose and dosing schedule once-a-day or twice-a-day dosing of rivaroxaban that will be safe and effective in preventing adverse cardiovascular outcomes such as death, myocardial infarctions MI including repeat myocardial infarction reMI, stroke, or ischemia inadequate blood supply to a local area requiring revascularization ie, the re-establishment of blood supply to a part or an organ in patients with ACS who are receiving antiplatelet therapy ie, aspirin alone or aspirin plus an approved thienopyridine, a type of drug such as clopidogrel that acts to inhibit the formation of blood clots. Approximately 3500 patients are planned to participate in the study for approximately 7 months. At study entry, all patients who are currently receiving treatment for ACS with antiplatelet therapy will be permitted to continue this therapy during the study. Patients will be enrolled and randomized to receive placebo, rivaroxaban administered as a once-daily dose, or rivaroxaban administered as a twice-daily dose at each dose level of rivaroxaban tested. Patients randomized at each dose level will continue to receive the same treatment for 6 months. Near the end of enrollment at the first dose level, available safety and efficacy data from patients will be assessed by an Operations Committee before enrolling and randomizing additional patients to the next higher dose level of rivaroxaban. Increasing dose levels of rivaroxaban are planned; however, progression to each higher dose level will be at the discretion of the Operations Committee. Patient safety will be monitored by evaluating adverse events reported, results from clinical laboratory tests, findings from electrocardiograms ECGs and vital signs measurements, findings from physical examinations, and the number of patients with protocol-defined major or minor bleeding, or bleeding requiring medical attention. All patients will take study drug or placebo tablets orally by mouth twice daily for 6 months starting at an initial total daily dose of 5 mg. Both once- and twice-daily dosing regimens will be tested at each rivaroxaban dose level planned. 