Twenty-two patients with colonic arterio-venous malformations (AVM) are reported and 218 other cases in the English literature are reviewed. Of our patients, 16 (73%) underwent right hemicolectomy because of repeated episodes of rectal bleeding or iron-deficiency anemia. Ten (63%) of these patients have remained in good health with no further evidence of intestinal bleeding after surgery. In six (37%) recurrent episodes of rectal bleeding or iron deficiency anemia have gone unexplained despite thorough work-up including repeat arteriography. Our experience and that in the literature suggests: 1. Most AVM's (78%) occur in the cecum or right colon, the jejunum being the next most common site. 2. Patients with AVM's tend to be elderly with a strong history of cardiovascular disease. A potential association between aortic stenosis, intestinal bleeding, and AVM's is suggested. However, prospective studies are needed to confirm this relationship. 3. In younger patients AVM's tend to occur at atypical sites such as the small bowel. 4. Surgical removal of an AVM cannot be expected to be uniformly curative. The causes of recurrent bleeding after surgery include: incomplete resection of the initial AVM, occult AVM's missed on arteriography and left behind at surgery, and the appearance of new AVM's postoperatively. The cause of bleeding in many patients with recurrence however, remains undiagnosed despite repeat angiography.