Hereditary non-polyposis colorectal cancer (HNPCC) predisposes to cancers of the colon, endometrium and several other extra-colonic sites in the absence of premonitory physical stigmata (Muir-Torre syndrome excepted). Discovery of the several DNA mismatch repair genes (hMSH2, hMLH1, hPMS1, hPMS2) holds the potential for determining the cancer destiny of patients, theoretically in utero. Pre-symptomatic DNA testing is now possible in patients from HNPCC families and will be clinically available once inexpensive and simple tests for these germ-line mutations have been effected. Genetic counseling will be mandatory, given the myriad socio-psychological, insurance, and potentially other personal issues which may impact this knowledge. New findings in the pathology of HNPCC, particularly an increased frequency of interval cancers and the likely accelerated rate of the adenoma to cancer sequence, indicate the need for more frequent colonoscopic surveillance with an option for prophylactic subtotal colectomy in germ-line-positive individuals.