The aim of this article is to examine the onset and clinical correlates of substance use in patients with psychotic disorders. One hundred and eight inpatients and outpatients with DSM-IV psychotic disorders were evaluated with the SCI-SUBS, an instrument designed to explore the spectrum of substance use and its clinical correlates. Comparisons were made between subjects with (n=47) and without (n=61) a DSM-IV diagnosis of substance use disorder (SUD). In patients with an early onset of psychosis (<17 years), the onset of SUD was subsequent. Patients with SUD had higher substance sensitivity, higher sensation-seeking traits and were more likely to self-medicate than patients without SUD. The reasons for self-medication endorsed by patients with SUD included relieving depression, achieving or maintaining euphoria, improving self-confidence and social abilities. Our results, based on a cross-sectional study, suggest that early onset of psychosis, substance sensitivity and sensation-seeking traits represent vulnerability factors for SUD. The relationships between SUD and psychosis should be examined systematically and clarified in longitudinal studies.