Up to 60% of chronic schizophrenic patients are reported to abuse alcohol or drugs. This comorbidity raises the question whether one disorder is a consequence of the other. With the structured interview "IRAOS," the onset and course of schizophrenia and substance abuse were retrospectively assessed in a representative first-episode sample of 232 schizophrenic patients. Information by relatives validated the patients' reports. Alcohol abuse prior to first admission was found in 24%, drug abuse in 14%-twice the rates in the general population. Alcohol abuse more often followed than preceded the first symptom of schizophrenia. Drug abuse preceded the first symptom in 27.5%, followed it in 37.9%, and emerged within the same month in 34.6% of the cases. The study demonstrates a remarkable association between first-episode schizophrenia and substance abuse, but a unidirectional causality is not supported, nor is a specific psychotic disorder in comorbid cases.