Colorectal cancer (CRC) is one of the most frequent cancers in Western countries. The identification of individuals at risk and the early diagnosis of CRC are of critical importance since a large proportion can be prevented or cured by surgical removal before metastasis has occurred. With increasing understanding of the genetic basis of hereditary and sporadic (non-hereditary) CRC, it becomes feasible to detect genetic alterations by molecular techniques. Familial adenomatous polyposis (FAP), hereditary nonpolyposis colorectal cancer (HNPCC), as well as early stages of spontaneous CRC, can be diagnosed by molecular characterisation of the adenomatous polyposis coli (APC) gene, the RAS oncogene and other genes in DNA from peripheral blood, stool or intestinal biopsies. With a better understanding of the genetic events leading to malignant transformation, molecular population screening should allow us to identify individuals at risk as well as patients with an early and potentially curable CRC. At present, careful patient and family history, physical examination and testing for occult blood as well as colonoscopy are still the key elements for clinical patient management. Molecular diagnosis will hopefully soon complement these analyses and should result in a reduction of morbidity and mortality from CRC.