Although a large proportion of the population contract cancer, the risk is not uniform. In a small group (an estimated five per cent) with hereditary predisposition, the risk of cancer is high (often as high as 50 per cent) in every child of an affected parent. Another, considerably larger group consists of individuals at high risk (though how high is unknown), where a combination of hereditary and environmental factors is probably involved. In all likelihood, it is this group which would benefit most from general recommendations as to preventive measures. The study of such individuals will enhance our knowledge of the source of their vulnerability, thus facilitating the understanding of tumorigenic mechanisms in general, and on the basis of such knowledge improve the prospect of general and specific screening programmes, prevention and treatment.