The incidence and nature of structural brain dysfunction was investigated in a pilot study sample of individuals currently assigned the Nebraska Penal Code designation of mentally disordered sexual offender. It was hypothesized that the sexual offenders would show a significantly higher incidence of dysfunction than a psychosocially normal group as evidenced by computed tomography scan measures, regional cerebral blood flow, and neuropsychological instruments. The hypothesis was tested by two different methods. One method used a 160 by 160 printout of density numbers generated by computer from computed tomography scans administered to the sexual offenders. The second method used mean blood flow data generated from a Harshaw TASC-5 Regional Cerebral Blood Flow analysis system. Additionally, the Luria-Nebraska Neuropsychological Test Battery was administered as a measure of the behavioral correlates of brain dysfunction. Preliminary analyses indicate that 50% of the sexual offenders tested showed brain dysfunction as demonstrated by decreased density measures, decreased blood flow, and performance deficits on the Luria Battery. The implications of these findings, if confirmed, are substantial on issues of criminal responsibility, sentencing, treatment, and rehabilitation of the sex offender. Case reports of the patients studied are presented.