The term hereditary non-polyposis colorectal cancer (HNPCC) was introduced initially to encompass autosomal dominant syndromes predisposing to colorectal cancer other than the polyposes. The term is a poor descriptor and is often applied to families on the basis of inadequate information. It is suggested that 'hereditary mismatch repair deficiency syndrome' (HMRDS) should replace the term HNPCC for describing the specific autosomal dominant condition which predisposes to cancer displaying the mutator phenotype. Population-based studies have shown that HMRDS probably accounts for no more than 2% of bowel cancer. A working diagnosis of HMRDS can be made on the basis of clinical, pathological and molecular characteristics. The histopathologist has an important role to play in the recognition and diagnosis of HMRDS. The characteristic morphology of colorectal cancer in HMRDS is reviewed and the diagnostic utility of 'field changes' and adenomas is discussed critically.