Effective prevention of cancer in patients with a hereditary disposition to malignant tumours was made possible by intensive prevention programs and molecular diagnosis. Taken hereditary non-polyposis colorectal cancer (HNPCC) as an example this article deals with the pathogenesis and molecular diagnosis in hereditary dispositions to cancer. HNPCC is inherited in an autosomal-dominant fashion and caused by germline mutations in genes responsible for detection an removal of DNA-basepair-mismatches (DNA-mismatch-repair-genes). The error rate in DNA replication is reduced thousandfold by these genes. A defective DNA-mismatch-repair results in tumours if the increased mutation rate causes alterations of tumour-suppressor- or oncogenes. HNPCC patients develop colorectal cancer but also tumours of the renal pelvis, the ureter, the small bowel, the endometrium and less often in other organs. The clinical presentation of these tumours may be characteristic, the clinical diagnosis may be guided by different clinical criteria catalogues. The suspicion is proven by the identification of a germline mutation in DNA-mismatch-repair-genes. This laborious diagnostic procedure is often preceded by prescreening procedures as the detection of microsatellite instability or immunohistochemical tests. Once the germline mutation is identified in a affected family member, the first degree relatives may be tested for this mutation. If they have inherited the mutation, they harbour a extremely high risk for developing cancer and therefore may be included in prevention programs. This so called predictive testing must be preceded by genetic counseling.