This study describes the treatment of severe obsessive-compulsive disorder in a 14-year-old male with an experimental single case design. Onset of OCD was at age 12. He had been hospitalized for 6 months the year before consulting. Almost all life spheres were affected by OCD. An initial behavioural intervention, which targeted tooth brushing, was used as a springboard to understand OCD and the process of change was framed within a cognitive account of OCD. The approach was then applied to a number of different targets within a unified framework. There was a 46% reduction in Y-BOCS score at post-treatment and 67% reduction through to 11-month follow-up. Points discussed include the involvement of other professionals, the changing role of the family as treatment progressed and as the patient started to seek more autonomy, and the choice of the initial target.