In order to determine the prevalence of psychiatric comorbidity in a population of young heroin users recruited from outside of the healthcare context, a sample was assembled by targeted sampling and nomination techniques; it was comprised of regular current users of heroin aged between 18 and 30 years and resident in Barcelona, Spain. Psychiatric evaluation was done with the Psychiatric Research Interview for Substance and Mental Disorders (PRISM) semi-structured interview. Of 149 individuals evaluated, 33% were women, whose mean age was 25.1 years; 93% received a diagnosis of heroin dependence and 71% of cocaine dependence. Thirty-two percent of the subjects had never been treated for substance use. Around two-thirds (67.1%, 95% CI: 59.6-74.7%) of the sample had lifetime psychiatric comorbidity, with antisocial personality and mood disorders being the most frequent conditions (33% and 26%, respectively). Mood, anxiety and eating disorders were more common among women than men. There were no differences in ever having been in treatment for drug use according to the presence of psychiatric comorbidity, although comorbidity was lower among those currently in treatment. Young heroin users recruited on the street presented a high prevalence of psychiatric comorbidity which was unrelated to past treatment history.