<qa>  What is the ratio of patient number of each group?From the end of May 2004 to the end of Dec. 2004, 720 patients with previous cerebral infarctionsee the inclusion criteria were enrolled in to the study and received one of the two treatment regimens, Cilostazol or Aspirin, the ratio of patient number of each group is 1:1. For each patient, the chance of entering either of these two groups is the same. The treatment will continue till the end of 2005. During the treatment period, patients will be observed concerning some certain events, mainly reoccurrence of stroke. If the patient experiences reoccurrence of stroke, or other event that the doctors think it is not appropriate to continue the study medication, this patient would stop the treatment. Patients were also required to take MRI head scan before entering the study and on completion of the treatment. 