Primary hypochondriasis is a poorly understood condition for which there is no accepted effective treatment. A cognitive-behavioural formulation is described which identifies the psychological processes and factors involved in the aetiology and maintenance of the condition. Using this formulation, it is also possible to account for the development and course of health concerns in non-hypochondriacal individuals. Modification of the aetiological and maintaining factors identified by the cognitive-behavioural formulation should provide an effective treatment for primary hypochondriasis. Cognitive-behavioural treatment strategies derived directly from the formulation have been used successfully in case-studies and require further systematic evaluation.