(Risperidone responders will be eligible to enter the four-month extension study. Placebo responders and risperidone non-responders will be managed as clinically appropriate by each research site.) Phase II: Extension Study - Four-month, open treatment with risperidone. Dose adjustment permitted according to clinical assessment (efficacy or adverse events). Two-month, randomized, double-blind, placebo-controlled discontinuation, parallel group design.