Angiodysplasia of the colon with preferential localisation in the region the ileocoecal valve is not infrequently the cause of massive gastrointestinal bleeding. The diagnostic problems of this disease are illustrated by seven cases. Contrary to angiography, coloscopy, which may have to be done as an emergency procedure, frequently does not allow localisation of the haemorrhage. Vascular malformations can be demonstrated by arteriography even in the asymptomatic interval. They frequently escape macroscopic identification even in the resected specimen due to their small size and their localisation in the submucosa. The diagnosis can be established histologically by serial sections with vascular filling if necessary. The treatment of choice is resection of the affected part of the gut. In cases of decreased operability endoscopic laser coagulation can be indicated.