The normal small intestine in man usually harbours a sparse microflora consisting of Gram-positive microorganisms derived from the oro-pharynx. The concentrations in the upper jejunum are approximately 10(3)-10(4) organisms/ml, whereas in the ileum the concentrations may be higher and faecal type organisms may also be present, such as Enterobacteria, Bifidobacteria, and Bacteroides in concentrations of 10(5)-10(6) organisms per ml of intestinal aspirate [24, 25]. This situation is well maintained unless the integrity of the small intestine is deranged. There are however, certain conditions in which bacterial proliferation in the lumen of the small intestine may occur and give rise to various metabolic abnormalities [2]. In this paper I would like to list some of these conditions, then describe the type of microflora which colonizes the small intestine, and finally discuss a few aspects of the metabolic consequences of these bacteria.