Colorectal cancer with microsatellite instability has a characteristic clinicopathologic profile, featuring right-sided, lymphocyte-rich tumors with a better prognosis than microsatellite stable (MSS) carcinoma. Mucinous and signet ring cell carcinomas are both over-represented among microsatellite instability-high cancers. The clinicopathologic features of mucinous microsatellite instability-high cancer parallel those of the overall microsatellite instability-high set, but it is not known whether the same is true for signet ring cell carcinoma, particularly given the fact that signet ring histology is a well-documented adverse prognostic factor. We recorded age, sex, tumor size, site, grade, stage, histologic pattern, growth pattern, Crohn-like reaction, vascular invasion and tumor-infiltrating lymphocytes in 72 resected signet ring cell carcinomas of the colorectum. Microsatellite instability was determined by a combination of polymerase chain reaction and immunohistochemical stains for hMLH1, hMSH2 and hMSH6. Tumors with instability at >30% of informative markers and/or loss of hMLH1 or hMSH2 expression were designated microsatellite instability-high; all others were classified as MSS. A total of 22 (31%) signet ring cell carcinomas were microsatellite instability-high. Compared to MSS signet ring cell carcinoma, they were more likely to be right-sided (81 vs 45%, P=0.005) and to affect older patients (68 vs 26%, P=0.0007) of female sex (59 vs 28%, P=0.03). Crohn-like reaction (45 vs 16%, P=0.02) and high tumor infiltrating lymphocyte counts (32 vs 8%, P=0.03) were more common in the microsatellite instability-high setting. There was no significant difference in 5-year survival in microsatellite instability-high vs MSS patients (41 vs 34%, P=0.3). In conclusion, approximately one-third of signet ring carcinomas of the colorectum are microsatellite instability-high. Microsatellite instability-high signet ring carcinomas share clinicopathologic features with other microsatellite instability-high cancers: older age group, female preponderance, right-sided location, Crohn-like reaction and numerous tumor-infiltrating lymphocytes. Microsatellite instability status does not appear to be a significant predictor of survival in signet ring cell carcinoma of the colorectum.