Between 1975 and 1984, 270 patients underwent a potentially curative resection for colorectal carcinoma. One hundred eighty-eight patients (70%) underwent preoperative colonoscopy, of which 129 patients (69%) were followed up with at least two postoperative colonoscopies. In 91 patients (70%), preoperative colonoscopy revealed no synchronous adenomatous polyps or cancer. Synchronous adenomatous polyps were found in 35 patients (27%), and three patients (2.3%) had a synchronous invasive cancer. Nineteen (54%) of the 35 patients with synchronous adenomatous polyps developed metachronous adenomatous polyps during the follow-up period compared with 24 (26%) of 91 patients without synchronous adenomatous polyps. The median interval to the development of metachronous adenomatous polyps was 19 months, and all of these polyps were 1 cm or less in size. Patients with synchronous adenomatous polyps less than 30 cm from the primary lesion (68%) developed metachronous polyps more often than did patients whose synchronous adenomatous polyps were more than 30 cm from the primary lesion (37%). Preoperative colonoscopy is important for determining synchronous pathology and identifying patients at risk for metachronous polyps.