Prolonged diarrhoea is rarely caused by an infection unless the patient is immunocompromised. Possible causative agents include giardia, Clostridium difficile and Entamoeba histolytica. Perform primary investigations for the diagnosis of the most common malabsorption disorders (coeliac disease, lactose intolerance) and refer the patients to colonoscopy to diagnose inflammatory bowel diseases (ulcerative colitis, Crohn's disease, microscopic colitis ). Keep the possibility of malignancy in mind if diarrhoea has not lasted a relatively long period of time. Colonoscopy is the first-line investigation if a malignancy is suspected. It is important to identify the irritable bowel syndrome (IBS ) because a proportion of these patients have (from time to time) diarrhoea. IBS symptoms may also appear after an infectious gastroenteritis. Faecal impaction may cause so-called overflow diarrhoea especially in elderly persons who are in institutional care. In these patients, the use of antidiarrhoeal medication may lead to confusion. Faecal impaction is verified by digital rectal examination and by plain abdominal x-ray. It is important to distinguish between diarrhoea and faecal incontinence (by assessing the resting tone and the contractile force of the anal sphincter, by identifying rectal prolapse on exertion). Identify and treat the complications of prolonged diarrhoea in elderly patients (dehydration, electrolyte disturbances, confusion, orthostatic hypotension, malnutrition).