This is a 4-week study to evaluate the safety and effectiveness of topiramate compared to placebo in the treatment of Bipolar I Disorder with an optional 6-month open-label (OL) extension for qualified patients following completion of the study. On Days 1-28 patients will receive placebo or topiramate 2x/day by mouth except for the 1st and last doses which will be a single evening dose and a single morning dose, respectively. Study drug will be titrated in 100-mg increments to 400 mg/day and patients maintained on a stable dose through Day 28. During the OL extension phase, topiramate will be titrated over 5 days to 200 mg/day. After Day 7, topiramate may be tapered down to 100 mg/day or up to 600 mg/day, as clinically indicated.