Participation in this study will last 12 weeks. All participants will first undergo baseline assessments that will include questionnaires and interviews about AN symptoms, a physical exam, vital sign measurements, an electrocardiograph (EKG), and a blood draw. Participants will then be assigned randomly to 12 weeks of treatment with daily olanzapine or aripiprazole. Participants will meet with a study doctor weekly over the 12 weeks of treatment. During these visits, the study doctor will monitor participants' progress, medication dosage, vital signs, and side effects. In addition, participants will undergo repeat blood draws every 4 weeks and repeat questionnaires every month of the treatment period. Upon completing the 12 weeks of treatment, participants will repeat most baseline assessments.