Some patients with the hereditary nonpolyposis colorectal cancer (HNPCC) syndrome develop carcinoma despite surveillance. The aim of this study was to determine whether survival was greater in colorectal cancer (CRC) cases detected by surveillance than in patients who had disease diagnosed on the basis of symptoms. All 150 CRC cases detected in 57 HNPCC families over the last 15 years were divided into two groups depending on whether they had been included in the surveillance program (n = 35) or not (n = 115). The stage distribution of the tumors in the group that underwent surveillance (Dukes' A, 50%; B, 35%; C, 15%; D, 0%) was significantly more favorable (P < .001) than in the group without surveillance (Dukes' A, 17%; B, 50%; C, 16%; D, 17%). CRC-specific 10-year survival was 93% in the surveillance group, significantly better than the 68% in the nonsurveillance group (P < .02). The overall survival did not differ significantly between the two groups despite a tendency in favor of the surveillance group. Colonoscopic surveillance enables early detection of CRC in HNPCC and reduces CRC mortality.