Endocrine and neurological abnormalities can affect an individual who may have a satisfactory socialization and no psychological stressors, such that he or she may be impelled to act out sexually. This conclusion is supported by most biological research and clinical findings on paraphiliacs. For example, some research studies of testosterone levels in violent individuals, including sexually violent persons, have been reported. Similarly, conditions such as temporal lobe epilepsy, brain tumors, and disturbances in the limbic system of the brain have been associated with paraphilic behavior. Sex offenders present both biological and psychological factors that make their clinical assessment and treatment complex. The task of relating biological factors to paraphilia is particularly complicated by the intricate nature of the brain and the endocrine system. The study of paraphilia has been limited predominantly to males. Brain damage and dysfunction in paraphilic men are examined, as well as chemical and other abnormalities in the endocrine system. It is concluded that sex offender evaluation should encompass sexual preference and history, substance abuse, mental illness, violence, neuropsychological factors, and endocrine abnormalities. 30 references