- The authors sought to determine the prevalence and effects of medical and psychiatric comorbidity on initial outcome in a group of patients experiencing a first episode of psychosis.Patients with a first episode of psychosis who were consecutively admitted to a hospital (N = 102) were examined for the presence of psychiatric and medical disorders. Patients were given psychiatric diagnoses with the use of the Structured Clinical Interview for DSM-III-R and were rated weekly on symptom rating scales. Outcome variables at discharge were final symptom rating scale scores, length of hospitalization, and recovery on the basis of operationalized criteria.Comorbid diagnoses were present in 52.0% (N = 53) of the patients, and 37.7% (N = 20) had multiple comorbid diagnoses. The most common comorbid diagnosis was substance abuse. Patients with affected psychoses were significantly more likely than those with nonaffective psychoses to have a comorbid substance abuse diagnosis. Patients with psychiatric comorbidity had poorer initial outcomes, while those with medical comorbidity had fewer symptoms at discharge.Comorbidity is common and may be a useful predictor of the outcome of a first episode of psychosis.