Paliperidone is being developed as a new therapeutic agent for the treatment of schizophrenia. The ER formulation of paliperidone was developed to deliver paliperidone at a relatively constant rate over a 24-hour period to improve the tolerability profile and decrease the potential for orthostatic hypotension. This study is designed to evaluate the efficacy and safety of 3 fixed dosages of paliperidone ER (6, 9, and 12 mg/day) compared with placebo in patients with schizophrenia. This trial is a multicenter, double-blind (neither the patient nor the physician will know if placebo or drug is being given and at what dose), randomized (patients will be assigned to different treatment groups based solely on chance), placebo- and active-controlled, parallel-group, dose-response study. The study includes a screening period of up to 5 days, followed by double-blind treatment phase of 6 weeks. Following the double-blind treatment phase, eligible patients (those who have completed the 6-week double-blind phase or who discontinue due to lack of efficacy after a minimum of 21 days) may enter a 52-week open-label extension phase with paliperidone ER monotherapy. Patients will be randomly assigned to 1 of 5 treatments (paliperidone ER 6, 9, or 12 mg, olanzapine 10 mg, or placebo) and will take oral dosages of assigned treatment once daily during the 6-week double-blind period. At the time patients enter the double-blind period, they must be inpatients, and must remain in the hospital for a minimum of 14 full days. While patients are hospitalized, efficacy will be assessed twice during the first week and at the end of the second week. After patients are discharged from the hospital, they will return to have efficacy and safety assessments performed on a weekly basis through the end of the 6-week double-blind period. The efficacy response will be measured by the change in the PANSS total score from start of treatment to the end of the double-blind phase. Safety will be monitored throughout the study and includes assessments of the incidence of adverse events; measurement of extrapyramidal symptoms using 3 rating scales (Abnormal Involuntary Movement Scale [AIMS], Barnes Akathisia Rating Scale [BARS], Simpson-Angus Rating Scale [SAS]); measurement of vital signs (lying down and standing blood pressure, pulse, temperature); electrocardiograms; and clinical laboratory tests. Double-blind: 6-, 9-, or 12-mg fixed dose of paliperidone ER, olanzapine 10 mg or placebo taken orally once daily for 6 weeks.