This is a single center, 16-week, randomized, double-blind, placebo-controlled, parallel group, flexible-dose study in 60 outpatients with schizophrenia, schizoaffective disorder, schizophreniform disorder, delusional disorder, psychotic disorder NOS, or bipolar disorder types I, II, or NOS by DSM-IV-TR criteria43 with a BMI > 22 for whom treatment with olanzapine (5-20 mg/day) would be appropriate as monotherapy or adjunctive therapy. Subjects who meet entry criteria will be randomized to treatment with olanzapine plus zonisamide or olanzapine plus placebo. All subjects will receive Personal Wellness Solution Counseling (http://www.zyprexa.com/hcp/hcp_patient_c_solutions_print.jsp). Both before and after randomization to zonisamide or placebo, patients will not be permitted to have any other major psychotropic medications (antipsychotics, mood stabilizers, antidepressants, or anxiolytics) added to their medication regimens. The primary outcome measure will be change in weight. Secondary outcome measures will include the Young Mania Rating Scale (YMRS),44 the Inventory for Depressive Symptoms (IDS),45 the Positive and Negative Symptoms Scale (PANSS),46 the Clinician Global Improvement (CGI) scale,47 the Binge Eating Scale (BES) 48, BMI, waist circumference, and metabolic variables (fasting lipids, glucose, insulin).