Previous case-control studies have suggested that carriers of monoallelic germline mutations in the MYH gene may be at increased risk of colorectal cancer. We applied a kin-cohort design, using a modified segregation analysis, to estimate the colorectal cancer risk using 300 first-degree relatives of 39 colorectal cancer cases who were monoallelic or biallelic carriers of MYH mutations. We found that monoallelic carriers had a 3-fold increased risk of colorectal cancer (hazard ratio, 2.9; 95% confidence interval, 1.2-7.0; P = 0.02) and biallelic carriers a 50-fold increased risk (hazard ratio, 53; 95% confidence interval, 14-200; P < 0.0001). This analysis illustrates the potential of family analysis to estimate cancer risk for low-frequency mutations and, based on the proportion of relatives predicted to be carriers, we believe that this constitutes the largest study of monoallelic carriers to date.