Previous epidemiological and clinical studies have shown high rates of substance use disorders in patients with psychotic disorders. There are few studies from the Scandinavian countries. The aim of this study was to investigate the rate of substance use disorders in a group of Norwegian psychotic inpatients from a specific catchment area. Sixty patients, aged 18-40 years, were interviewed through standardized methods: the Addiction Severity Index (EuropASI) and the Structured Clinical Interview for DSM-IV axis I disorders (SCID-I). Urine toxicology screens confirmed patients' self-report of recent substance use. The lifetime rate of substance use disorders was 70% when all psychotic disorders were included and 62.5% when substance-induced psychotic disorders were excluded. Fifty percent of all the patients studied had current substance use disorders. The majority of substance use disorders were dependence disorders. Alcohol, amphetamine and cannabis were the dominant substances. The level of comorbidity found in this study is comparable with that found in American studies, despite lower prevalence of substance use in the Norwegian population. The high rate of substance use disorders in psychotic inpatients has implications for the treatment and the organization of psychiatric care for these patients.