Phase 1: Subjects may enter Phase 1 within the first 12 weeks of treatment with olanzapine. At entry into Phase 1, anthropometric measurements will be taken (height, weight, waist and hip circumference) and BMI calculated. Baseline assessments will include questionnaires about hunger (to be done in the fasting state) (Subjective Satiety Scale), and activity (Baeckea Activity Questionnaire), respiratory quotient (RQ) and resting energy expenditure (REE) assessed using the Medgraphics metabolic cart, and body composition determined by dual x-ray absorptiometry (DXA, Hologic Delphia). Prior to every DXA scan Body Composition Lab personnel will administer a safety questionnaire and women of reproductive age will have a blood pregnancy test. Visits will be scheduled weekly to monitor weight and to provide healthy lifestyle counseling based on the Solutions for Wellness program developed by Lilly. During the first 3 weeks, subjects will be called by research personnel once per week to complete a 24 hour diet recall by phone. During the first 4 weeks of the study, RQ and capillary glucose will be repeated weekly since it is anticipated that changes to these parameters may occur early on. At monthly intervals clinical interviews will monitor for changes in psychosis and blood pressure will be assessed. When subjects gain 1 BMI unit, fasting bloodwork and baseline assessments are repeated and they are enrolled in Phase 2. Subjects who do not gain 1 BMI unit within 12 weeks of olanzapine treatment will end study participation at the end of Phase 1.