Twenty-six right-handed patients from the Schizophrenia Research Unit of the New York State Psychiatric Institute, New York, and 26 normal controls were tested. The patients met criteria of the DSM-IV35 for schizophrenia (undifferentiated, n=11; paranoid, n=5; disorganized, n=3); schizoaffective disorder (bipolar type, n=4); or schizoaffective disorder (depressive type, n=3). Data for the patients with schizophrenia and schizoaffective disorder were pooled for the analyses reported in this article. When analyses of the ERP data were repeated after excluding patients with schizoaffective disorder, all patient vs control differences for N1, N2, and late positivity reported below for the full sample remained statistically significant. The patient group included 16 men and 10 women who ranged in age from 20 to 55 years, with a mean (SD) age of 33.2 (10.9) years, and had a mean (SD) education level of 13.3 (3.1) years. Research diagnoses were made on the basis of information provided from clinical interviews and from a semistructured interview by a trained and reliable rater using the Diagnostic Interview for Genetic Studies.36 This interview schedule includes the items from other commonly used instruments (eg, Structured Clinical Interview for DSM-III-R, Patient Edition,37 The Scale for the Assessment of Positive Symptoms,38 and The Scale for the Assessment of Negative Symptoms39). A consensus diagnosis was made by the rater and a senior clinician (X.A.). At the time of testing, 18 of the patients were receiving antipsychotic medications: 4 patients were receiving haloperidol (mean dosage, 13.1 mg/d; range, 7.5-20 mg/d), 3 patients were receiving thiothixene (mean dosage, 18.3 mg/d; range, 15-20 mg/d), 2 patients were receiving fluphenazine hydrochloride (mean dosage, 25.0 mg/d; range, 20-30 mg/d), 2 patients were receiving perphenazine (mean dosage, 46 mg/d; range, 32-60 mg/d), 1 patient was receiving trifluoperazine hydrochloride (40 mg/d), 3 patients were receiving risperidone (mean dosage, 5.3 mg/d; range, 3-9 mg/d), 2 patients were receiving clozapine (mean dosage, 512.5 mg/d; range, 450-575 mg/d), and 1 patient was receiving olanzapine (20 mg/d). The remaining 8 patients (4 with schizophrenia and 4 with schizoaffective disorder) did not receive antipsychotic medications for an average of 4 weeks (mean, 28.1 days; range, 13-61 days) prior to testing.