A subgroup of children with childhood-onset OCD and/or tic disorders share a common clinical course characterized by dramatic onset and symptom exacerbations following scarlet fever or strep. throat infections. Such infections may be prevented by the prophylactic (preventative dose) administration of antibiotics, such as penicillin. This study will determine the effectiveness of penicillin prophylaxis in preventing relapses of OCD and/or tics in the PANDAS subgroup.