The presence of a bidirectional risk for metachronous carcinomas among women with thyroid and breast cancer is well established. However, the underlying risk factors remain poorly understood. Two sisters developed papillary thyroid cancer (PTC) at age 32 and 34 years, followed by ductal carcinoma of the breast at 44 and 42 years. The 2 children of the younger sister developed ataxia-telangiectasia; the son also developed lymphoblastic lymphoma and his sister died secondary to acute lymphoblastic leukemia (ALL). They were found to be compound heterozygous for ataxia telangiectasia mutated (ATM) gene mutations (c.3848T>C, p.L1283P; and c.802C>T, p.Q268X). Exome sequencing of the 2 sisters (mother and aunt of the children with ataxia-telangiectasia) led to the detection of the pathogenic monoallelic ATM mutation in both of them (c.3848T>C; minor allele frequency [MAF] < 0.01) but detected no other variants known to confer a risk for PTC or breast cancer. The findings suggest that monoallelic ATM mutations, presumably in conjunction with additional genetic and/or nongenetic factors, can confer a risk for developing PTC and breast cancer.