Substance misuse (SM) (drug/alcohol dependence or abuse) in psychotic illness is an increasingly recognized problem. We aimed to estimate the prevalence and examine the influence of SM on age at onset of psychosis and psychopathology among patients with first-episode psychosis. One hundred seventy-one consecutive patients with first-episode psychosis were assessed. SM, age of onset of psychosis and psychopathology were determined using valid instruments. Seventy-seven (46%) patients had a lifetime history of SM and were predominately males, had more positive symptoms, and in the majority of cases (84%), started misusing substances before the onset of psychosis (SM-BP). There was no difference in age of onset between patients with SM-BP and the rest of the sample. Lifetime history of SM is common and may influence psychopathology, but does not appear to influence or bring forward the age at onset of psychotic symptoms. © 2009 The Authors. Journal compilation © 2009 Blackwell Publishing Asia Pty Ltd.