The aim of this study is to describe the psychological effects on a man with a mild learning disability who was sexually assaulted whilst in prison, and the treatment which was carried out. J. suffered Post-Traumatic Stress Disorder (PTSD), as defined in DSM-III-R, combined with associated features of depression and anxiety. Treatment involved imaginal exposure to the assault, followed by systematic de-sensitization to the traumatic experience of imprisonment preceding the assault. Activities were also carried out to develop J.'s self-esteem, coping strategies and anxiety management. The successful outcome of treatment is discussed in relation to models of emotional processing. Some discrepancies between scores on self-evaluation questionnaires and behavioural observations raise issues concerning the best way of assessing treatment outcome for people with learning disabilities.