Hospitalization is required for at least the first 7 days of the acute treatment phase. At the beginning of the acute treatment phase, patients will be randomly assigned to receive ER paliperidone or olanzapine in a 4:1 ratio. Patients in the ER paliperidone group who have a clinical response at the end of the acute treatment phase, remain clinically stable throughout the continuation phase, and achieve remission for each of the last 3 weeks of the continuation phase will again be randomly assigned: they will be assigned in a 1:1 ratio to receive ER paliperidone or placebo in the maintenance phase. Patients in the olanzapine treatment group who fulfill the same criteria will continue receiving double-blind treatment with olanzapine in the maintenance phase.