Crohn's disease (CD) and tuberculosis (TB) of the gastrointestinal tract pose major diagnostic problems for clinicians where these conditions coexist. Clinically and radiologically, the diseases are similar. In the West, TB is considered in the differential diagnosis of all suspected cases of CD, particularly among Asian migrants. Earlier age of presentation, perianal disease and enteric fistulae favour a diagnosis of CD. Aphthoid ulceration, pseudopolyps and filliform mucosa at endoscopy are suggestive of CD and a negative tuberculin test is useful. The final diagnosis depends largely on histopathology and the presence or absence of acid fast bacilli. Tuberculosis is more common in developing countries and intestinal TB frequently coexists with pulmonary tuberculosis. TB is known to affect all age groups and fistulous communication, although uncommon, does occur. In addition to radiology and endoscopy, laparotomy may be required to establish the diagnosis. In developing countries, CD is uncommon and remains largely a diagnosis of exclusion. A trial of anti-tuberculosis therapy may often be prescribed before definitely diagnosing CD. The development of molecular biology techniques has led to a revival of interest in mycobacteria as a possible aetiological agent in CD. DNA from Mycobacterium paratuberculosis and Mycobacterium kansaii have both been identified in CD cases but the significance of this finding has not been established. However, in the near future polymerase chain reaction will become increasingly useful in differentiating CD from intestinal TB because it allows the amplification and identification of very small quantities of mycobacterium DNA.