This article describes the assessment and treatment of Leo, a young boy who was seen in twice weekly psychotherapy for three years. The first part of the article presents the material obtained during the assessment process organized by following the framework of the 2001 version of the Provisional Diagnostic Profile (Davids et al., 2001). The second part addresses the main themes arising from the boy’s treatment and reflects back on the initial diagnostic impressions following the assessment phase. We seek to illustrate the value of a thorough and detailed diagnostic framework in contrast to the limitations presented by the sole use of descriptive diagnostic criteria based on symptomatology.