Attention Deficit and Hyperactivity Disorder (ADHD) is highly prevalent amongst criminal justice populations: ADHD diagnoses and medication are upwards of five times more likely for individuals who have a Swedish prison record than those without. We merge Swedish prison registers (with detailed healthcare data) to out-of-prison healthcare, crime, and employment data to study the effect of in-prison ADHD diagnoses. For individuals with no ADHD history, this new diagnosis treatment can include an information shock, medication and/or therapy. We compare the pre- and post-prison dynamics for treated inmates to alternative undiagnosed comparison groups: all untreated individuals, untreated early spells for repeat offenders treated in later spells, or a matched and reweighted control group. A robust set of findings emerge. New in-prison ADHD diagnoses significantly and persistently increase post-prison ADHD and substance abuse related healthcare. Crime and labor market outcomes, however, do not improve. There are also significant family spill-over effects: both children and siblings with no previous history of ADHD are more likely to be treated for ADHD after a newly diagnosed family member’s prison spell. Though prison appears to serve as an institution to bring high-risk, vulnerable populations into the public healthcare system, our results suggest that ADHD related care may not be as effective at lowering crime as many policy makers argue.