Abnormalities affecting the pancreas or small intestine can result in malabsorption and diarrhoea by interfering with either the degradative or absorptive phases in the handling of one or more nutrients. Diet may be involved in the pathogenesis of certain disorders and can also influence the nature and severity of the clinical manifestations of gastrointestinal disease. Exocrine pancreatic insufficiency can have serious nutritional consequences as the pancreas plays a crucial role in the degradation of the main dietary constituents. Whereas gastrointestinal peptidases help compensate for pancreatic protease deficiencies, malabsorption of carbohydrate and fat occurs and can result in osmotic and secretory diarrhoea, respectively. Small intestinal disease can result in malabsorption by interference with either the number or functioning of individual enterocytes. Diarrhoea may be due to stimulated secretion and diminished absorption of fluid and electrolytes, and also to the osmotic effects of small molecules, particularly carbohydrates, that remain in the lumen. Specific therapy depends on the disease. Dietary management is also important, for example with a restricted fat diet containing highly digestible carbohydrate and a high quality protein source. Where a specific dietary sensitivity is suspected, such as gluten-enteropathy, a hypoallergenic diet may prove particularly valuable.