A possible association between Mycobacterium avium ssp. paratuberculosis ( Map) and Crohn's disease was first suggested in 1913, when similarities between the gross pathology and symptoms of Johne's disease in cattle and those of Crohn's disease in humans were first noted. Crohn's disease is now generally believed to have a multifactorial aetiology with genetic predisposition, environmental factors ( infectious agent, diet or smoking), and abnormal inflammatory response all playing a part. Evidence supporting a link between Map and Crohn's disease includes: higher detection rates of Map by PCR and culture in gut samples from Crohn's patients compared with controls; demonstration of a serological response to Map antigens in Crohn's patients; and anti-Map antibiotic therapy resulting in remission, or substantial improvement in disease condition, in many patients. The available scientific evidence has been reviewed by a number of expert groups in recent years. The consensus opinion, at present, is that the available information is insufficient to prove or disprove that Map is a cause of Crohn's disease, but the hypothesis is still plausible. The recent discovery of a susceptibility gene in Crohn's patients, NOD2/ CARD15, does not preclude a role for Map in the pathogenesis of at least some cases of Crohn's disease, as the function of this gene is bacterial sensing in the gut. If Map does contribute to the causation of Crohn's disease then it may not be acting as a conventional infectious agent.