Men with schizophrenia are at increased risk, as compared to the general population, for criminal offending and to have displayed Conduct Disorder (CD) before age 15. The present study examined the consequences of CD among a sample of 248 men with schizophrenia or schizo-affective disorder, aged, on average, 39 years old. Participants were intensively assessed at discharge from the hospital and four times during the subsequent two-year period. CD was associated with criminality and substance misuse among first-degree male relatives and substance misuse among female relatives. In childhood and adolescence, CD was associated with poor academic performance, substance abuse, and physical abuse. In adulthood, the diagnosis of CD and each CD symptom were associated with increased non-violent and violent criminal offending, after adjusting for life-time diagnoses of substance misuse disorders. CD was not associated with homicide. CD was associated with life-time diagnoses of alcohol and drug abuse and/or dependence. During the 24 month follow-up period, CD and the number of CD symptoms were associated with aggressive behavior, controlling for life-time diagnoses of substance use disorders, substance misuse measured objectively and subjectively, medication compliance, and obligatory care. CD was associated with an earlier age at onset of schizophrenia and at first admission to hospital, and with length of time spent in hospital. During the two-year follow-up period, neither the diagnosis of CD nor the number of CD symptoms was associated with levels of positive and negative symptoms assessed five times, compliance with medication, substance use, or readmission. The results are interpreted to suggest that CD is a distinct co-morbid disorder that runs parallel to the course of schizophrenia.