Despite high rates of attention-deficit/hyperactivity disorder (ADHD) among adult lawbreakers, particularly the long-term effects of ADHD pharmacotherapy remain unclear, not the least because of ethical challenges with preventing control subjects in randomized controlled trials from receiving medication over prolonged time. We followed up adult male prisoners with ADHD who completed a 5-week randomized, double-blind, placebo-controlled trial followed by a 47-week open-label extension of osmotic-release oral system methylphenidate in a Swedish high-security prison from 2007 to 2010 (ClinicalTrials.gov: NCT00482313). Twenty-five trial completers were prospectively followed up clinically 1 year (24/25, 96% participated fully or in part) and 3 years (20/25, 80% participation) after trial regarding ADHD symptoms (observer and self-reports), psychosocial functioning, substance misuse, and criminal reoffending. Methylphenidate-related improvements in ADHD symptoms and psychosocial functioning obtained during the 52-week trial were maintained at 1- and 3-year follow-ups. Specifically, after 3 years, 75% (15/20) of the respondents had been released from prison, and 67% of these (10/15) had employment, usually full time. In contrast, nonmedicated respondents at the 3-year follow-up (5/20) reported more ADHD symptoms, functional impairment, and substance misuse compared with currently medicated respondents (15/20). Further, 40% of the respondents self-reported reoffending, indicating a substantially lower relapse rate than expected (70%-80%).In summary, although these observations need validation from new and larger samples, positive effects were maintained after 4 years of methylphenidate treatment. Most study completers were employed and had no relapse in substance misuse or criminality. These results suggest that motivational support and continued medication are important for improved outcome in adult criminal offenders with ADHD.