The role of primary genetic factors in the etiology of cancer has become of intense interest to the research and clinical community. This interest has been heightened by recent discoveries that germ-line mutations such as BRCA1 and BRCA2 in hereditary breast cancer are responsible for an increasing percentage of common solid tumors. A potpourri of proto-oncogenes and tumor-suppressor genes has been identified in hereditary as well as certain common sporadic and rare cancer types, and new cancer genes will likely be discovered every month to account for the 5 to 10% of the cases of cancer that can be attributed to primary genetic factors. Molecular mechanisms in the pathogenesis of hereditary cancer can result in more-targeted cancer-control measures. At least four mutator genes (MHS2, MLH1, PMS1 and PMS2) appear to account for 70-80% of hereditary nonpolypoid colorectal cancer (HNPCC). When one of these germ-line mutations is present in an HNPCC family, the physician is then able to determine the patient's lifetime cancer destiny with an accuracy of about 90% (limited only by the penetrance of the gene). This will enable highly targeted surveillance to be initiated early, such as colonoscopy beginning at ages 20 to 25 or prophylactic subtotal colectomy. Also, in multiple endocrine neoplasia syndromes (MEN 2A and 2B), the identification of the culprit RET proto-oncogene now enables a secure diagnosis and permits testing of children who might benefit from prophylactic total thyroidectomy. Central to translation of these momentous molecular genetic discoveries into patient care is the necessity of determining who requires DNA testing. The cancer family history is the linchpin in making this decision.