Young adult male offenders in Japanese correctional institutions were questioned confidentially about their history of sexual abuse. The self-reported information showed that when the sexual abuse went undetected, the relationships between abuser and victim were closer, and more victims were reported to be under the influence of substances. At the same time, the nature of the sexual abuse was even more serious among self-reported-only (undetected) offenders in terms of direct sexual acts. In terms of unhealthy sexual preference and childhood sexual victimization, sexual abusers reported higher level of problems comparing to nonsexual abusers. Self-reported-only sexual abusers identified even more problems than officially identified sexual offenders. The results suggest that when we focus only on the characteristics shown by officially identified sexual offenders, we may overlook important risk and need factors that should be addressed in psychotherapy for preventing future reoffending.