Cognitive behaviour therapy (CBT) treatments have been developed and validated with respect to specific diagnoses. In routine clinical practice diagnostic accuracy is poor, making for poorly targeted treatment. The problems posed by lack of diagnostic rigour, including non-detection of co-morbidity, are rarely the subject of supervision sessions and treatment failures may be inappropriately attributed to other factors such as lack of therapeutic skill or an unmotivated client. It is argued that a false dichotomy exists between diagnosis and case formulation fuelled by professional territorial disputes. We suggest that diagnosis acts as a lens, focusing attention on the range of cognitions salient to a case formulation and also highlights psychosocial and environmental factors that may affect treatment outcome. It is recommended that practitioners enhance their effectiveness by using structured interviews routinely as a part of their ongoing assessment of clients.