Of all forms of cancer, approximately 5% are caused by factors leading to a strong genetic predisposition. DNA diagnosis is currently used in families with hereditary tumour syndromes, such as familial adenomatous polyposis, hereditary non-polyposis colorectal carcinoma (Lynch syndrome), and hereditary breast and ovarian cancer. Those persons who have not inherited the predisposition no longer have to undergo regular examinations. DNA diagnosis for a hereditary predisposition is currently also performed in patients with cancer at a relatively young age, even if the family history is unclear or negative. Consideration of the patient in the context of his or her family is important for both medico-technical and psychosocial reasons. This is true of both diagnostic and presymptomatic DNA diagnosis. For these reasons, the clinical application of the DNA diagnosis of hereditary tumours has become an integral part of the work of the multidisciplinary cancer family clinics of the university medical centres and the cancer centres. Guidelines for the management of hereditary tumours have recently been issued, with criteria for referral to the specialised outpatient clinics.