Antipsychotic agents have, for a long time, been used to alleviate the severe behavioral problems associated with manic episodes. Risperidone, widely used in the treatment of schizophrenia, has been shown to be effective in the treatment of manic and mixed episodes associated with bipolar disorders. This is a randomized, double-blind study to evaluate the effectiveness and safety of risperidone compared with placebo after 3 weeks of treatment in patients with bipolar disorder who are experiencing manic episodes. A secondary objective is to estimate the difference between the anti-manic efficacy of risperidone and haloperidol (active comparator) after 12 weeks of treatment. Haloperidol also serves as an internal control for the 3-week treatment period. The study has two 2 phases: an acute period consisting of 3 weeks of double-blind treatment (risperidone, haloperidol, or placebo) followed by a maintenance period consisting of 9 weeks of double-blind treatment (risperidone or haloperidol) or 9 weeks of open-label treatment (risperidone). Patients receive study medication to be taken orally once a day at gradually increasing doses during the first week (risperidone, a range of 1 - 6 mg/day or haloperidol, a range of 2 - 12 mg/day) to achieve optimal effectiveness, while minimizing any intolerance to the drug. Daily treatment continues at the optimal dose through Week 3 of the first phase. After completing the 3-week double-blind period, patients can continue double-blind treatment for an additional 9 weeks at the optimal dose (with placebo patients crossed over to risperidone), or enter the 9-week open-label period of risperidone treatment. Adjustment to achieve an optimal dosage is made for those patients whose medication is changed upon entering the second phase.