Background: Patients diagnosed with schizophrenia display poor premorbid adjustment (PPA) in half of the cases. Attention deficit/hyperactivity disorder (ADHD) and conduct disorder (CD) are common child psychiatric disorders. These two facts have not previously been linked in the literature. Aims: To determine the prevalence of ADHD/CD problems retrospectively among patients with psychoses, and whether and to what extent the high frequency of substance abuse problems among such patients may be linked to ADHD/CD problems. Method: ADHD and CD problems/diagnoses were retrospectively recorded in one forensic (n = 149) and two non-forensic samples (n = 98 and n = 231) of patients with a psychotic illness: schizophrenia, bipolar or other, excluding drug-induced psychoses. Results: ADHD and CD were much more common among the patients than in the general population—the odds ratio was estimated to be greater than 5. There was no significant difference in this respect between forensic and non-forensic patients. Substance abuse was common, but substantially more common among patients with premorbid ADHD/CD problems. Conclusions: Previous views regarding PPA among patients with a psychotic illness may reflect an association between childhood ADHD/CD and later psychosis. The nature of this association remains uncertain: two disorders sharing some generative mechanisms or one disorder with two main clinical manifestations. Childhood ADHD and particularly CD problems contribute to the high frequency of substance abuse in such groups.