Establishing a clear definition of diarrhoea based on history alone can prove difficult, and this tends to lead to over investigation of functional bowel disorders such as IBS. However, in the majority of such patients typical symptoms and negative initial investigations yield a positive diagnosis. In those patients where there is doubt, inspection of the stool and measurement of stool weight may prove helpful in clarifying whether there is true "diarrhoea" or functional disease. Initial investigations should include full blood count, erythrocyte sedimentation rate, electrolytes, liver function tests, iron studies, vitamin B12, folate, and thyroid function. Screening tests such as serum antiendomysium antibodies for coeliac disease, the most common small bowel enteropathy in European populations, should be performed early in the course of investigations. The initial assessment should direct the clinician to determine whether further investigation is necessary and, if so, whether the focus should be on colonic, small bowel, or pancreatic disease. This analysis could reasonably be performed in the primary care setting. Most chronic diarrhoea is due to colonic disease, and in the absence of clinical evidence for malabsorption, investigations should focus on the lower gastrointestinal tract in the first instance. Colonic investigations should be age stratified, in keeping with the risk of neoplasia. This risk is greater in those with one or more first degree relatives with colorectal cancer and in males. Full colonic imaging is recommended in those over 45 years of age, preferably with colonoscopy. In patients younger than this the diagnostic yield of flexible sigmoidoscopy and biopsy is not substantially different from colonoscopy, and sigmoidoscopy can therefore be the preferred investigation. Patients with malabsorption represent a small proportion of presentations with chronic diarrhoea. Supporting history may direct investigations towards either the small bowel or pancreas. Serological testing for coeliac disease will determine most cases without invasive investigation, but individuals suspected to have small bowel malabsorption, despite negative coeliac serology, should have endoscopic distal duodenal biopsies taken to exclude other rarer forms of small bowel enteropathy. This strategy has largely supplanted many of the older tests of small bowel function. Non-invasive tests for pancreatic insufficiency currently depend on the presence of at least moderate impairment of exocrine function before they achieve adequate sensitivity. There are indirect functional tests such as the BTP/PABA and Pancreolauryl tests, and more direct tests of pancreatic enzymes (such as elastase or chymotrypsin) in the stool. Although the sensitivities and specificities are similar, faecal elastase is preferred because of its ease of use. These tests are reasonably specific for pancreatic malabsorption and are preferred over tests for faecal fat, which do not adequately discriminate between small bowel and pancreatic malabsorption. The three day faecal fat is often unreliable in clinical practice and is no longer recommended. The optimal investigation for small bowel bacterial overgrowth remains unclear. Culture of jejunal aspirates or unwashed small bowel biopsies remains the gold standard and should be encouraged whenever the diagnosis is seriously considered. The sensitivity of hydrogen breath tests is only approximately 60%, with little difference between 14C-D-Xylose and glucose. Their specificity is approximately 75%, which is better than for lactulose hydrogen breath testing. Given the apparent deficiencies in the current methods for establishing diagnoses of pancreatic insufficiency, bile acid malabsorption, and small bowel bacterial overgrowth, an empirical trial of therapy is often employed. The diagnostic value of this approach has not been subject to evaluation. Despite extensive and exhaustive investigations, some cases will resist a definitive diagnosis. Although no study has formally assessed an investigative protocol for chronic diarrhoea such as that described, it is estimated that approximately two thirds of cases can be diagnosed using such an approach. The remaining patients will be those with watery, secretory, self limiting "idiopathic" diarrhoea (presumably infective), or undiagnosed factitious diarrhoea.13 23 213 Since in the majority of these cases the overall prognosis appears to be good, further investigation in this group is not warranted and symptomatic treatment should be instituted.