Background: Case formulation (CF) is considered a core component of cognitive behavioural therapy (CBT). Despite this, few studies have explored the content and quality of CFs in routine practice and any associated impact on treatment outcome. Aims: This study investigated (1) the content, timing and quality of CF for patients (N = 29) with Obsessive Compulsive Disorder; (2) the impact of CF on symptoms, distress and therapeutic alliance; and (3) the relationship between CF quality and treatment outcome. Method: CF content and quality were assessed from audiotapes of routine CBT sessions using a validated coding manual and evaluated against treatment outcomes at different stages of therapy. Results: CFs were developed early during treatment and contained strong behavioural and cognitive components, with a focus on symptoms and maintaining factors. CF quality ratings ranged from rudimentary to excellent. A significant improvement in distress and the therapeutic alliance occurred following the introduction of CF, but no significant relationship was found between CF quality and outcome at any stage of treatment. Conclusion: CF may be valuable in reducing patient attrition, due to its impact early in treatment. Further research is needed to explore the most important components of CF. Theoretically sound and disorder specific measurement tools for evaluating CFs are required.