In the past 2 years the major genes for inherited breast-ovarian cancer, bowel cancer and melanoma have been identified. Relatives of cancer patients frequently perceive themselves to be at high risk of cancer. Appropriate assessment of the family history permits the identification of those at particularly high risk who may benefit from special programs of prevention, screening and surveillance. In certain cases, genetic testing and prophylactic surgery may be indicated. This article looks at the general issues involved. A follow-up article next month will look at specific diseases.