The therapeutic assessment of a 12-year-old boy who sexually abused, is described. He was one of forty-eight boys who took part in the psychotherapeutic assessment stage of a large multi-disciplinary research project, funded by the Department of Health. For a period of some years, at a young age, ‘Frank’ was brutally sexually abused by his father. Having been severely traumatised, he became traumatising for others when he started at puberty to sexually abuse. The twelve-session assessment consisted of semi-structured interviews and standardised questionnaires within the framework of more open-ended psychotherapeutic consultations. The authors discuss the clinical and theoretical implications of this work.