This study investigates the cancer risks associated with germline mutations in DNA-mismatch-repair genes, specifically *MSH2*, *MLH1*, *PMS1*, *PMS2*, and *MSH6*. The research focuses on hereditary non-polyposis colorectal cancer (HNPCC) families, comparing the cancer incidence of mutation carriers to the general Finnish population. The study involved 1763 members of 50 HNPCC families, categorized based on their genetic status as mutation carriers, non-carriers, or individuals at 50% or 25% risk of being a carrier. The results show significantly increased standardized incidence ratios (SIR) for various cancers, including colorectal, endometrial, gastric, biliary tract, uro-epithelial, kidney, and central nervous system tumors. The cumulative incidence rates for colorectal and endometrial cancers were particularly high, reaching 82% and 60%, respectively, by age 70. The study highlights the need for targeted cancer prevention and screening strategies, especially for extra-colonic cancers, and suggests that endometrial cancer may be a more significant risk factor in women than colorectal cancer.This study investigates the cancer risks associated with germline mutations in DNA-mismatch-repair genes, specifically *MSH2*, *MLH1*, *PMS1*, *PMS2*, and *MSH6*. The research focuses on hereditary non-polyposis colorectal cancer (HNPCC) families, comparing the cancer incidence of mutation carriers to the general Finnish population. The study involved 1763 members of 50 HNPCC families, categorized based on their genetic status as mutation carriers, non-carriers, or individuals at 50% or 25% risk of being a carrier. The results show significantly increased standardized incidence ratios (SIR) for various cancers, including colorectal, endometrial, gastric, biliary tract, uro-epithelial, kidney, and central nervous system tumors. The cumulative incidence rates for colorectal and endometrial cancers were particularly high, reaching 82% and 60%, respectively, by age 70. The study highlights the need for targeted cancer prevention and screening strategies, especially for extra-colonic cancers, and suggests that endometrial cancer may be a more significant risk factor in women than colorectal cancer.