A proportion of cases of breast and colon cancers are caused by inherited mutations that confer a greatly increased susceptibility to malignancy. Certain clinical features may help distinguish patients with a genetic cause for their cancer from the larger number of patients with sporadic tumors. A thorough family medical history is also necessary to identify those at high risk for developing cancer. Many of the normal functions of these genes are understood, and mutation analysis for patients and their families is now available as a clinical service. Presymptomatic detection of mutations allows the patient to pursue preventive measures to reduce the probability of developing a malignancy. Evidence is now available that some prophylactic measures do reduce the incidence of cancer and reduce mortality in mutation carriers, and the standard of care is evolving rapidly. The essential elements necessary to provide accurate interpretations of molecular genetics test results to patients are described.