The primary measure of effectiveness (acute efficacy) is the change in Young Mania Rating Scale (YMRS) total score from baseline to the endpoint at 3 weeks of the acute treatment period. Additional efficacy measures for the study assess maintenance efficacy. These measures include the Clinical Global Impression-Severity of Illness (CGI-S) scale; Global Assessment Scale (GAS), which assesses the patient's level of functioning; Brief Psychiatric Rating Scale (BPRS), a scale for measuring psychotic symptoms; and the Montgomery Asberg Depression Rating Scale (MADRS), which evaluates symptoms of depression. Safety assessments include the incidence of adverse events and measurement of vital signs (temperature, pulse, blood pressure) throughout the study; 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) at the start and end of both phases of the study. The study hypothesis is that 3 weeks of daily treatment with risperidone is more effective than placebo, as measured by Young Mania Rating Scale scores, in the treatment of the manic phase of Bipolar I Disorder. Acute phase: Risperidone orally, once-daily: 2 mg on Day 1, 1 - 3 mg on Days 2 to 4, and 1 - 6 mg on Days 5 to 21; or haloperidol orally, once-daily: 4 mg on Days 1 to 4 and 2 - 12 mg on Days 5 to 21; or placebo orally, once-daily Days 1 to 21. Maintenance phase: optimal dose of risperidone or haloperidol for 9 weeks (placebo patients cross over to risperidone) or Open-label phase: risperidone for 9 weeks.