['The aim of this study was to examine the expression of mismatch repair (MMR) proteins in Lynch syndrome (LS)-associated colorectal adenomas and to evaluate their relationship with clinicopathological variables and potential utility in LS screening.', 'We performed immunohistochemistry for MLH1, PMS2, MSH2 and MSH6 in 134 adenomas obtained from 26 genetically confirmed LS patients. MMR deficiency, as determined by loss of any MMR protein, was observed in 113 adenomas (84%). All the MMR-deficient adenomas exhibited homogeneous loss of MMR proteins, which reflected underlying germline mutations. MMR deficiency was more frequent in adenomas obtained from older patients (aged ≥60 years; 81 of 86, 94%), with larger tumour size (>5 mm; 71 of 73, 97%) and with high-grade dysplasia (50 of 51, 98%). Multivariate analyses indicated that increased age and larger tumour size were associated independently with MMR deficiency.'] Is mismatch repair deficiency in Lynch syndrome-associated colorectal adenomas more prevalent in older patients?