Attention Deficit Hyperactivity Disorder (ADHD) has long been recognized in children, and for many the disorder persists into adulthood. There is a growing concern that the adults with ADHD who have the least favorable outcome, are among those who end up in prison. The aim of this study was to assess childhood ADHD and its persistence into adulthood among a representative sample of Norwegian prison inmates, as well as personality disorders and reading difficulties, which in previous studies have been linked to ADHD. The results indicate that persistent ADHD is very common among prison inmates. Personality disorders and reading difficulties are also common. Psychiatric comorbidity complicates the diagnosis of ADHD in adults. A greater awareness about ADHD in adults certainly is warranted, especially within the prison system because of the risk of misdiagnosing psychiatric disorders and also the risk of missing a condition possibly amenable to treatment.