Different aspects of the small bowel bacterial overgrowth syndrome are reviewed. Special emphasis is put on the newly recognized structural and functional abnormalities of the small intestinal mucosa, abnormalities that may not be fully reversed by effective antimicrobic therapy. The pathogenetic mechanisms involved in the malabsorption of different substances are discussed and the available diagnostic tests are briefly presented. The current therapy, surgical, medical and supportive, are outlined. It is pointed out that abnormal overgrowth flora of the small intestine can occur unassociated with malabsorption. Thus, the clinician must assess the potential benefit to be derived from treatment, once the presence of absorptive abnormalities is documented.