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 desensitization 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.