Subjects will be aged 10 to 17 years with a diagnosis of Bipolar I disorder and suffering from a current mixed or manic episode. On enrollment, subjects will be assigned to receive 1 of 3 treatments (oral placebo tablets, oral risperidone tablets 0.5 to 2.5 mg, or oral risperidone tablets 3 to 6 mg), which will be administered daily for 3 weeks. Study medication will be increased to the target dosage during the first 7 days, then further increased within the target dosage range reached until maximum tolerated dose is reached by day 10. The maximum tolerated dose will be given for the last 12 days of the study. Risperidone (0.5 to 2.5 mg or 3 to 6 mg doses) or placebo given orally as 0.25, 0.5, 1, 2, 3, or 4 mg tablets (or matching placebo) each day for 3 weeks.