• The number of patients with recent ACS taking rivaroxaban with background antiplatelet therapy who experience protocol-defined clinically significant bleeding. [ Time Frame: Day 1 to Day 210 ] [ Designated as safety issue: Yes ]
  

• The number of patients who experience death (all cause), MI, (or reMI), stroke, or ischemia requiring revascularization [ Time Frame: Day 1 to Day 210 ] [ Designated as safety issue: No ]
  
• The number of patients who experience death (all cause), MI (or reMI), or stroke [ Time Frame: Day 1 to Day 210 ] [ Designated as safety issue: No ]
  
• The number of patients who experience death (all cause), MI (or reMI), stroke, severe recurrent ischemia requiring revascularization, major or minor bleeding to assess net clinical benefit [ Time Frame: Day 1 to Day 210 ] [ Designated as safety issue: No ]
  
• The number of patients who experience death (all cause), MI (or reMI), stroke, or severe recurrent ischemia [ Time Frame: Day 1 to Day 210 ] [ Designated as safety issue: No ]
  
• The number of deaths (all cause) [ Time Frame: Day 1 to Day 210 ] [ Designated as safety issue: No ]
  

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.