OBJECTIVES: In an effort to increase understanding of the relationship between sexual violence and mental illness, we conducted an investigation of the prevalence of psychiatric illness and associated psychological features of sexual offenders. METHODS: Twenty-two male sex offenders were referred for psychiatric diagnosis from the prosecutor's office and admitted to the hospital between August 2010 and May 2011. In order to identify Axis I and II disorders, including sexual disorders, we conducted the Korean version of the Structured Clinical Interview for Diagnostic and Statistical Manual of Mental Disorder, Fourth Edition. All participants completed psychological assessment batteries, including the Korean-Wechsler Adult Intelligence Scale, and self-report scales, including the Barratt Impulsiveness Scale, State-Trait Anger Expression Inventory, and the Rape Myth Acceptance Scale. Information on participants' legal, personal, and psychiatric family histories was also gathered. RESULTS: Many of the participants received diagnosis of Axis I disorders : Eight subjects (36.4%) had paraphilia, and five (22.7%) had pedophilia. Two subjects (9.1%) had schizophrenia, five (22.7%) had depressive disorder, and three (13.6%) had alcohol dependence. Participants also showed high rates of Axis II disorders : four of them (18.2%) had mental retardation or borderline intellectual functioning and seven (31.8%) had cluster B personality disorders. Participants' serum free testosterone level showed statistically significant correlation with motor impulsiveness (p<0.05). CONCLUSION: Sex offenders included in this study displayed high rates of mental illness, including paraphilia and intellectual disability. On the basis of our findings, we asserted that sex offenders should undergo careful evaluation for mental illness, and that management programs for sex offenders should include psychiatric evaluation and intervention.