The small intestine is the principal organ of absorption in the body, thus complete resection is not compatible with life (Doolan et al., 1951). However extensive small bowel resections are, for a time at least, compatible with survival (McMahon, 1966; Walker-Smith & Wyndham, 1967a), but cause a profound disturbance of digestion and absorption. The three main functions of the small bowel are: (1) the onward passage of the ingested food bolus, (2) continued digestion of this bolus, (3) absorption of the digested nutrients into the blood and lymph vessels. Thus, disease of this organ may manifest as a disruption of one or all three of these functions. Interference with the first produces the syndrome of intestinal obstruction and disorders of the second and third usually produce diarrhoea and malabsorption. In childhood, intestinal obstruction due to congenital abnormalities of the small bowel usually presents acutely and requires urgent surgical treatment, whilst disorders of digestion and absorption may be more insidious in onset and occult in their clinical manifestations. Indeed, although disorders of digestion and absorption usually manifest with diarrhoea and malabsorption, significant disease of the small bowel may be present in children, as biopsy studies have shown, in the absence both of diarrhoea and of evidence of malabsorption by the usual clinical screening tests. If such abnormalities remain unrecognized, they may gravely interfere with growth and development. The introduction of safe methods to biopsy the small bowel mucosa in childhood (Sakula & Shiner, 1957; Crosby & Kugler, 1957; Cameron et al., 1962) has enabled knowledge of disease of this organ to be considerably extended in the paediatric age-group. Nevertheless, much remains unknown, and although a single biopsy may reveal proximal small bowel * Wellcome Clinical Research Fellow. abnormality, this gives no indication as to the extent of the abnormality along the gut. The purpose of this paper is to review several aspects of some predominantly medical diseases of the small bowel which may occur in childhood. The selection of conditions discussed reflects experience at the Royal Alexandra Hospital for Children, Sydney, Australia.