Diarrhoea is a frequent symptom in ulcerative colitis (UC) and Crohn's disease (CD) which it often reveals. Its clinical features, course, physiopathology and treatment are different in these two diseases. UC is accompanied by an imperative, blood-stained, diurnal and often noctural diarrhoea the intensity of which fluctuates with the course of the disease, usually regressing under medical treatment but capable of becoming severe. This diarrhoea is due to several physiopathological mechanisms such as loss of colonic absorption function, exudative enteropathy and disorders of motility. Contrary to the lesions of UC, those of CD involve the whole digestive tract; they are multisegmental, ileal, colonic, ileocolonic or diffuse. Diarrhoea takes different forms depending on the site and evolutive stage of the lesions: malabsorption of bile salts and/or fatty acids in ileal lesions or after surgical resections, bacterial proliferation above stenoses, intestinal shunts due to internal fistulae, reduced absorbing area, exudative enteropathy and motor disorders in colonic lesions. In UC, treatment of the diarrhoea is that of exacerbations: systemic or topical corticosteroids or 5-aminosalicylic acid. In CD, side by side whit the maintenance treatment of the disease with anti-inflammatory drugs, the symptomatic treatments of diarrhoea play an important role and must be tailored to the responsible physiopathological mechanisms; some patients need artificial feeding.