This paper explores the genesis of sadistic behaviour in men and its relationship to crime. Sixteen male special hospital patients, each with a diagnosis of psychopathic disorder, formed the basis of this descriptive study. In only three cases were the crimes explicable in terms of external circumstances and personality traits. The offences of the remaining 13 cases became comprehensible only when the offender's internal circumstances were explored: investigation revealed repetitive sadistic masturbatory fantasies which had spilled over into overt behaviour because the patients had felt impelled to seek and create increasingly dangerous in vivo 'try-outs' of their fantasies. The paper discusses the crucial link between sadistic fantasy and behaviour.