Ileoanal anastomosis has evolved as a true alternative to proctocolectomy with permanent ileostomy over the past decade and has made a major impact on the operative treatment of patients with ulcerative colitis and familial polyposis. Experimental studies and clinical experience support the addition of a proximal ileal reservoir to the ileoanal anastomosis. Several types of reservoirs have been proposed of which the U-shaped reservoir has found the widest acceptance and the most frequent application. This reservoir constitutes an adequate replacement for the rectum by providing a low-pressure, compliant, peristaltically quiescent reservoir which still can be triggered to evacuate spontaneously at a reasonable threshold volume. Overall, the ileoanal anastomosis with proximal ileal reservoir fulfills the set clinical requirements: the underlying disease (polyposis, ulcerative colitis) is dealt with in a curative manner. An ileostomy is avoided. Anal continence is preserved. The operative procedure is safe and of comparable mortality and morbidity to the proctocolectomy with ileostomy. It is reasonable to expect that this procedure, i. e. ileoanal anastomosis with proximal ileal reservoir, will become the operation of choice for patients with ulcerative colitis and familial polyposis.