The authors examined the role of fantasy as an internal drive mechanism for repetitive acts of sexual violence. A sample of 25 serial sexual murderers with three or more known victims each was compared with a sample of 17 single sexual murderers, with only one known victim each. The drive mechanism was hypothesized to be an intrusive fantasy life manifested in higher prevalences of paraphilias, documented or self-reported violent fantasies, and organized crime scenes in the serial murderers. All three hypotheses were supported.