DNA mismatch repair genes are responsible for the condition hereditary non-polyposis colorectal cancer (HNPCC). Genomic destabilization caused by failure of DNA mismatch repair leads to the progressive accumulation of somatic mutations and so to accelerated oncogenesis. The aim of this study was to document the rate of background mutational activity in the normal colorectal mucosa of subjects with HNPCC. A naturally occurring model utilizing a genetic polymorphism (O-acetyltransferase) allows the development of unicryptal loss of heterozygosity (LOH) to be detected by means of mild PAS histochemistry and quantified. The rate of unicryptal LOH was measured in informative and affected members of 3 HNPCC families and found to be within the expected range. This result is consistent with the finding that normal cells in HNPCC subjects are DNA repair proficient and supports the view that the mutational effects of the HNPCC gene occur selectively within adenomatous epithelium and serve to accelerate the adenoma-carcinoma sequence.