Aim: Clinical staging of schizophrenia entails a new method that identifies clusters of symptoms and variation in level of remission, with the goal to create a framework for early intervention. Additionally, duration of untreated psychosis (DUP) may influence symptom severity in the first episode of psychosis (FEP) and could necessitate refining of the staging model. However, consistent evidence concerning variation in symptom severity and DUP between stages is missing. Therefore, we evaluated the clinical validity of the staging model by investigating differences in symptom severity across stages in schizophrenia spectrum disorders. Second, we assessed if a prolonged DUP is associated with higher symptom severity in FEP. Methods: We performed a cross-sectional study of 291 acutely admitted patients with a schizophrenia spectrum disorder. Patients were assigned to clinical stages following the definition of McGorry. Symptom severity was evaluated with the new DSM-5 Clinician-Rated Dimensions of Psychosis Symptom Severity (CRDPSS). In FEP, we determined the DUP. Results: Significantly higher severity scores of CRDPSS items hallucinations (H = 14.34, df = 4, P-value =.006), negative symptoms (H = 19.678, df = 4, P-value =.001) and impaired cognition (H = 26.294, df = 4, P-value = <.001) were found in more advanced stages of disease. Moreover, patients with FEP and a DUP longer than 1 year showed significantly more severe negative symptoms (U = 314 000, P =.015) compared to patients with a DUP shorter than 1 year. Conclusions: The present study found supporting evidence for the clinical validity of the staging model in schizophrenia spectrum disorders. In addition, we found support for refining the stage “first episode” with information concerning the DUP.