Background—Up to 15% of colorectal cancers are characterised by DNA microsatellite instability (MIN), shown by the presence of DNA replication errors (RERs). Aims—To identify pathological features that are discriminating for colorectal cancer (CRC) showing extensive MIN. Subjects—A prospective series of 303 patients with CRC and no family history of either familial adenomatous polyposis or hereditary non-polyposis colorectal cancer. Methods—DNA was extracted from fresh tissue samples and the presence of MIN was studied at nine loci that included TGFβRII, IGFIIR, and BAX. The 61 cases showing RERs were compared with 63 RER negative cases with respect to a comprehensive set of clinical and pathological variables. Predictive utility of the variables was tested by decision tree analysis. Results—Twenty seven patients with CRC showed extensive RERs (three loci or more) (RER+) and 34 had limited RERs only (28= one locus; 6 = two loci) (RER+/−), yielding a bimodal distribution. RER+ cancers differed from RER− and RER+/− cases. Tumour type (adenocarcinoma, mucinous carcinoma, and undifferentiated carcinoma) (p=0.001), tumour infiltrating lymphocytes (p=0.001), and anatomical site (p=0.001) were the most significant of the discriminating variables. Algorithms developed by decision tree analysis allowed cases to be assigned to RER+ versus RER− and +/− status with a global sensitivity of 81.5%, specificity of 96%, and overall accuracy of 93%. Conclusion—Pathological examination of CRC allows assignment of RER+ status; assignment is specific and relatively sensitive. Conversely RER− and RER+/− CRC are indistinguishable.