This study uses the Grid of Problematic States (GPS) to examine Lisa's case, one of the most successful in the York Psychotherapy Depression Project. This study tried to assess whether the contents of mental experience form stable clusters consistent with a diagnosis of depression. It was possible with the GPS to pinpoint problematic states typical of depression and trace the transitional states occurring in Lisa between two different mental states: depressive and well-being. The GPS analysis suggested that the treatment successfully managed to deal with symptoms and to change the patient's thought themes and emotions. At the end of treatment, Lisa was less sad and displayed some anger, and a state of being nurtured emerged.