['The inherited susceptibility to hereditary nonpolyposis colorectal cancer (HNPCC) provides an opportunity for secondary prevention of colorectal cancer (CRC) in family members who are at 50% lifetime risk. The aim of this study was to evaluate the effectiveness of long-term screening during a 10-year period.', 'The CRC and death rates were compared between two groups of asymptomatic at-risk members of 22 families with HNPCC: 133 subjects screened at 3-year intervals by colonoscopy or barium enema and sigmoidoscopy and 118 control subjects without screening. The screening was complete in 118 subjects (89%), whereas 18 control subjects (15%) had screening examinations outside of the study.', 'CRC occurred in 6 study subjects (4.5%) and in 14 controls (11.9%; P = 0.03), a difference of 7.4% in favor of the study group, which corresponds to a reduction by 62% that is presumably because of polypectomies. The tumor stage was more favorable in the screening group with no deaths caused by CRC compared with 5 of 14 cases in controls. Overall, there were 6 and 12 deaths within the 10-year period in the study and control groups, respectively (P = 0.08).'] Does screening reduce colorectal cancer rate in families with hereditary nonpolyposis colorectal cancer?