This article traces the evolution of forensic psychiatry in England over the first half of the twentieth century, and the growing interest within this new field in sexual offenders. Taking as its focus the little-studied field of prison medicine, it explores the role of prison doctors in promoting the idea that crime was often indicative of psychological illness, and that sexual offenders in particular were profitable candidates for medical examination. Definitions of the sexual offence and the mentally disordered offender were profoundly influenced by underlying beliefs about what constituted both normal sexual behaviour for men, and normal criminal behaviour for women. Research into the treatment of sexual offenders was thus not only responding to new medical theories, but also reflecting broader social and penological change. Although prison doctors and forensic medicine remained marginal, this article highlights the durable connections between particular sexual crimes and mental disorder that were forged.