We studied 2332 individuals with monoallelic mutations in MUTYH among 9504 relatives of 264 colorectal cancer (CRC) cases with a MUTYH mutation. We estimated CRC risks through 70 years of age of 7.2% for male carriers of monoallelic mutations (95% confidence interval [CI], 4.6%-11.3%) and 5.6% for female carriers of monoallelic mutations (95% CI, 3.6%-8.8%), irrespective of family history. For monoallelic MUTYH mutation carriers with a first-degree relative with CRC diagnosed by 50 years of age who does not have the MUTYH mutation, risks of CRC were 12.5% for men (95% CI, 8.6%-17.7%) and 10% for women (95% CI, 6.7%-14.4%). Risks of CRC for carriers of monoallelic mutations in MUTYH with a first-degree relative with CRC are sufficiently high to warrant more intensive screening than for the general population. ; Aung Ko Win, James G. Dowty, Sean P. Cleary, Hyeja Kim, Daniel D. Buchanan, Joanne P. Young, Mark Clendenning, Christophe Rosty, Robert J. MacInnis, Graham G. Giles, Alex Boussioutas, Finlay A. Macrae, Susan Parry, Jack Goldblatt, John A. Baron, Terrilea Burnett, Loïc Le Marchand, Polly A. Newcomb, Robert W. Haile, John L. Hopper, Michelle Cotterchio, Steven Gallinger, Noralane M. Lindor, Katherine M. Tucker, Ingrid M. Winship, Mark A. Jenkins