Subjects were 36 convicted, incarcerated sexual murderers, 29 of whom had engaged in serial killings. Data also were collected on 118 of their victims. This was the full data set; random selection was not used. Data included psychiatric, criminal, pretrial, and prison records, court transcripts and interviews. Analysis of data indicates that objectively quantifiable differences in crime scene patterns could be used to distinguish organized from disorganized sexual murderers. Organized offenders were likely to plan out the crime, be in a skilled occupation, follow crime events in the media, be intelligent, have precipitating stress, and have been angry and depressed at the time of murder. Disorganized offenders were likely to know the victim, live alone, be sexually inhibited, and have been frightened and confused at the time of the crime. These offenders also were likely to come from backgrounds that included parental hostility and sexual problems, low-birth order, and unstable paternal employment. The majority of offenders showed a dysfunctional family background (bonding failures, psychological problems, physical abuse, substance abuse, sexual problems), early development of a violent sexualized fantasy life, and early development of exploitive, aggressive, and sexual behavior. Phases two and three of the study will examine differentiating variables in a larger sample and develop a model for profiling these offenders. Five references are given; tabular and other research data are appended.