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. Antipsychotic drugs like risperidone have also been used as therapeutic agents in the treatment of patients who are not responsive to mood stabilizers alone. This is a randomized, double-blind study to evaluate the effectiveness and safety of risperidone compared with placebo, as an addition to mood stabilizing drugs, in the treatment of patients experiencing manic episodes associated with bipolar disorder. The study has two phases: a double-blind treatment phase (3 weeks) and an open-label phase (10 weeks). To participate in the study, patients must be in-patients for a minimum of the first 4 days of double-blind treatment. During the double-blind treatment phase, patients receive risperidone or placebo tablets to be taken once a day at gradually increasing doses at investigator's discretion, up to a maximum dose of 6 mg/day, while continuing their treatment with a mood stabilizer (lithium, valproate, or carbamazepine). In the open-label phase, therapy with a mood stabilizer continues, and all patients receive risperidone with dosage gradually adjusted to achieve optimal effectiveness. The primary measure of effectiveness is the change in Young Mania Rating Scale (YMRS) total score from baseline to end of double-blind treatment. Additional efficacy measures include the Brief Psychiatric Rating Scale (BPRS), the Clinical Global Impression (CGI), (which evaluates the change in severity of the disorder), and the Hamilton Depression Rating Scale (HAMD). Safety assessments include the incidence of adverse events throughout the study; measurement of vital signs (pulse and blood pressure) and evaluation of the presence and severity of extrapyramidal symptoms by the Extrapyramidal Symptom Rating Scale (ESRS) at specified intervals; and clinical laboratory tests (hematology, biochemistry, urinalysis) before study initiation, at completion of the double-blind treatment, and at the end of the study. The study hypothesis is that daily treatment with risperidone as add-on therapy provides better effectiveness than the addition of placebo, as measured by Young Mania Rating Scale scores, in the treatment of the manic phase of bipolar disorder. Risperidone 1 mg tablets, taken orally, once daily; Doses of 2 mg on Days 1 and 2, up to 4 mg on Days 3 and 4, and up to 6 mg (maximum dose) on Days 5 through 21. Same dose maintained through the 10 week open-label phase. Gradual dose adjustments are allowed to achieve optimal effectiveness.