Complications involving the colon occurred in 28 of 325 patients who received renal transplants. Pseudomembranous colitis, the most common complication, affected 15 patients, two of whom required surgery. Three instances of diverticulitis were complicated by free perforation in two cases, and by colovesical fistula in one. Appendicitis occurred in two cases. The other complications were hemorrhage (from diverticulosis or angiodysplasia), nonspecific colitis, and ischemic colitis. Spontaneous colonic perforation did not occur. Two thirds of the colonic complications occurred within 30 days after transplantation. All cases of colon perforation, however, occurred later than one month after transplantation. It is concluded that pseudomembranous colitis is the most common colonic complication in renal transplant recipients, that it usually occurs early, and that it carries a good prognosis. Colonic perforations occurred later in this series and were treated successfully. All cases of ischemic colitis were part of terminal multiorgan system failure.