AIMS To examine the drug use and injection risk-taking among incarcerated methadone maintenance (MM) patients; to determine the impact of a diagnosis of antisocial personality disorder (ASPD) on prison-based MM treatment; to compare incarcerated patients with community patients. DESIGN Structured interview. SETTINGS New South Wales (NSW) prisons and community methadone maintenance (MM) units. PARTICIPANTS One hundred incarcerated MM patients and 183 community MM patients. MEASUREMENTS Subjects were interviewed about drug use and needle risk-taking in the previous 6 months, and assessed for a diagnosis of ASPD. FINDINGS Heroin had been used by 38% of prison MM patients in the 6 months prior to interview, on a median of 4.5 days. Forty-four per cent of prison patients had injected a drug in the preceding 6 months. A third (32%) of prison subjects had borrowed used injecting equipment within the preceding 6 months, and 35% had lent used injecting equipment to others. Community patients were more likely to have injected a drug in the preceding 6 months (84% vs. 44%), to have used heroin (72% vs. 38%) and to have done so more frequently (20 vs. 4.5 days). Prisoners, however, were more likely to have borrowed (32% vs. 15%) and lent (35% vs. 21%) injecting equipment in that time. While injecting at lower rates than their community counterparts, the injecting occasions of prisoners were of much higher levels of risk. A diagnosis of ASPD was unrelated to both drug use and needle risk-taking. CONCLUSIONS Incarcerated patients injected less frequently than community patients, but had higher levels of needle risk-taking.