Converging evidence indicates that self-disorders (SDs) selectively aggregate in schizophrenia spectrum conditions. The aim of this study was to test the discriminatory power of SDs with respect to schizophrenia and nonschizophrenia spectrum psychosis at first treatment contact. SDs were assessed in 91 patients referred for first treatment through the Examination of Anomalous Self-experience (EASE) instrument. Diagnoses, symptoms severity, and function were assessed using the Structural Clinical Interview for the DSM-IV, Structured Clinical Interview for the Positive and Negative Syndrome Scale, Calgary Depression Scale for Schizophrenia, Young Mania Rating Scale, and Global Assessment of Functioning-Split Version. Most patients found it highly relevant to talk about SDs. EASE total score critically discriminated between schizophrenia, bipolar psychosis, and other psychoses. The EASE total score was the only clinical measure that showed a significant and robust association with the diagnosis of schizophrenia. Systematic exploration of anomalous self-experiences could improve differential diagnosis in first-treatment patients.