Background: Patients with chronic low back pain present physicians with diagnostic and therapeutic problems. Physical treatments tend to have low success rates and it is postulated that this may be because low back pain can be a manifestation of abnormal illness behaviour. Methods: A structured prospective study determined the prevalence of somatization in a sample of 131 adult patients with chronic low back pain using the Illness Behaviour Questionnaire (IBQ) and the Modified Somatic Perception Questionnaire (MSPQ). The scores on these psychological questionnaires were compared with the blind interpretation of pain distribution drawings and with the results of a mechanical classification of the patient's symptoms and signs. Results: Fifty‐four per cent of patients had four or more (out of five) abnormal illness indicators. The MSPQ values for the group were significantly above the control values in the literature. Thirty‐two per cent of pain diagrams were thought to be incompatible with an organic cause when assessed by an orthopaedic surgeon and sixty‐two per cent when assessed by a psychiatrist. Conclusions: Psychosocial factors are dominant in the presentation of chronic low back pain in adults and the disorder is not primarily a musculoskeletal one.