The review of the development of theory and research on serial homicide shows the significant differences between researchers in explanations and typologies. The analysis of the 20 sexually sadistic serial murderers involved data from the FBI's National Center for the Analysis of Violent Crime; cases selected were those in which the offender had a repetitive pattern of sexual arousal to images of suffering or humiliation as well as other factors. The authors present a case analysis of the Hillside Stranglers, Angelo Buono and Kenneth Bianchi, so as to address one of the study problems; this involved reducing each of the 20 files to minimum estimates of the frequency of the features and some of the limitations in relying upon incomplete archival sources. White males were overrepresented in their sample, and some of the results challenge widely held assumptions about serial murderers' backgrounds. More than 50 percent of the sample was of above average intelligence, and 65 percent came from middle-class families. There was evidence of traumatic experiences in early life. More than one-half of the men engaged in several diverse paraphilias in addition to their interests in sexual paraphilias. Some 82 percent of the sample engaged in violent fantasies that were supported by some form of collection and/or expressive behavior. One of the most disturbing features of the crimes was the degree and level of planning and thoroughness of execution involved in the murders. The authors analyze why some individuals must bring their fantasy into reality by murdering while others in society do not. The research suggests that the sexually sadistic killer is a distinctive type of serial murderer whose expertise and thoroughness make him a particularly dangerous threat to society. 3 tables and 20 references