Angiodysplasia, a condition of unknown aetiology, is thought to represent the commonest cause of obscure gastrointestinal bleeding, particularly in the elderly population. The lesions of angiodysplasia, which are small (less than 5 mm) and usually multiple, consist of microvascular abnormalities in the mucosa and submucosa of the bowel wall. They are most commonly found in the caecum and right side of the colon, but have been observed in the stomach, ileum and elsewhere in the colon. The diagnosis is made by either selective visceral angiography and/or colonoscopy, but the lesions cannot be detected on barium studies or with the naked eye at laparotomy. Localisation of the abnormalities by the histopathologist is greatly facilitated by special injection techniques demonstrating the blood vessels of resected colonic specimens prior to fixation and section. Treatment may be conservative (iron replacement) if the anaemia is not severe, by endoscopic fulguration or by colonic resection. Other common causes of gastrointestinal blood loss should always be excluded before the final diagnosis of angiodysplasia is accepted. Although the condition is well recognised as a significant cause of gastrointestinal blood loss a number of questions remain to be answered. The true incidence of angiodysplasia in the population is not known, the aetiology of the condition remains a mystery, although many theories abound.