The purpose of this study is to determine if amoxicillin is an effective treatment for PANDAS, as well as examining possible mechanisms of this effect. We propose to do this with a 3-month double-blind placebo-controlled crossover trial of amoxicillin treatment with a randomized A-B-A-B design. Outcome measures will be blinded ratings of tic severity, standardized measures of psychopathology, and immunologic assays (such as type I and type II cytokine levels).