We report on a case, ulcerative colitis and another of Crohn's disease. During a relapse which was unresponsive to conventional therapy, acid-fast bacilli were found in colonic biopsies. Conventional therapy was substituted with antimycobacterial chemotherapy (rifampicin, isoniazid and ethambutol) which was responsible for a marked improvement. However, a relapse occurred during chemotherapy and no acid-fast bacilli were found. The patients became responsive to sulphasalazine and corticosteroid therapy once again. It appears that Mycobacteria play a collateral role in inflammatory bowel disease and that once they have been eliminated the original disease re-emerges.