['About 15% of colorectal adenocarcinomas have a deficient DNA mismatch repair phenotype. The frequency of deficient DNA mismatch repair tumours increases with age due to the hypermethylation of hMLH1 promoter. The study aimed to determine the prognostic value of deficient DNA mismatch repair phenotype in elderly patients.', 'Mismatch repair phenotype was retrospectively determined by molecular analysis in consecutive resected colorectal adenocarcinoma specimens from patients over 75 years of age from 4 Oncology centres.', '231 patients (median age: 81, range: 75-100) were enrolled from 2005 to 2008. Mean prevalence of deficient DNA mismatch repair phenotype was 22.5%, and 36% for patients over 85 years. Deficient DNA mismatch repair status was significantly associated with older age, female sex, proximal colon primary and high grade tumour. For stage II tumours no deficient DNA mismatch repair tumours had a recurrence at end of follow-up compared to 17% for tumours with proficient phenotype. The proficient phenotype status was significantly associated with worse age-adjusted overall survival [HR 2.60; 95% CI 1.05-6.44; p=0.039]. For stage III tumours a trend for less recurrence was observed for deficient DNA mismatch repair phenotype (16%) compared to proficient phenotype (36%).'] Is deficient mismatch repair phenotype a prognostic factor for colorectal cancer in elderly patients?