There is a need to better understand the sex killer and determine if there are ways to prevent the homicides they commit. Clinical data were examined on men that were sex killers, or engaged in some sexual acts with their victims and killed them before, during, or after the sexual acts. The purpose was to contrast sex killers with groups they appeared to resemble most and from which they may emerge -- non-homicidal sexual “aggressives” (men that sexually assault and/or rape women), sadists, and a general sample of sex offenders, to determine if they can be identified before they commit homicides. Thirty-three sex killers were compared to 80 sexual aggressives, 23 sadists, and 611 general sex offenders on sexual history and preferences, substance abuse crime, violence, mental illness, personality, and neurological and endocrine abnormalities. The results indicate that sex killers showed a number of features that distinguished them from other sex offender groups and that can serve as warnings to clinicians to provide more intensive treatment and/or closer supervision. They started their criminal careers at young ages and appeared to have been very disturbed children. Significant numbers were in reform school, or members of criminal gangs, and 39 percent committed the sex killing before the age of 20. There was more cruelty to animals, more fire setting, and other antisocial behaviors such as theft and vandalism than seen in other groups, especially in the general sample of sex offenders. Early signs of learning problems and neuropsychological impairment were most evident in the sex killers that more often failed grades in school and were in special education classes for children with learning problems. There were often behavioral problems in school. The sex killers showed a more frequent history of sadism as well as voyeurism, fetishisms, transvestitism, and gender identity disturbance. The abuse of alcohol and especially of drugs was more common among sex killers than any other group. 10 tables, 39 references