Identification of the microsatellite instability (MSI) phenotype in endometrial carcinoma is important given that such tumors are the most common noncolorectal tumors to occur in hereditary nonpolyposis colorectal cancer syndrome, and may bear prognostic relevance. The objective of this study was to assess the utility of immunohistochemistry (IHC), a simple and fast technique, in detecting MSI in endometrial carcinoma. The study subjects consisted of 90 endometrial carcinoma patients with equal representation of MSI-high (MSI-H) and non-MSI-H tumors. MSI was tested using the standard polymerase chain reaction-based method and the 5 NCI-recommended markers. Overall, IHC with MLH1 and MSH2 antibodies detected 69% of MSI-H tumors with a specificity of 100%. Adding PMS2 and MSH6 to the antibody panel increased the sensitivity to 91% but decreased the specificity to 83%. The most common IHC abnormality in MSI tumors was concurrent loss of MLH1/PMS2. Assessment of staining was straightforward in most cases but not in all. Staining inadequacies existed. Five stains (4 MLH1 and 1 MSH6) were not interpretable because of the lack of any internal positive control. Two percent to 10% of the cases (depending on the antibody assessed) had only focal weak staining; the highest frequency (10%) occurred with MLH1 antibody. PMS2 staining detected 7 MLH1-staining present MSI-H cases, thus partly accounting for the increased sensitivity with the 4-antibody panel. MSH6 staining identified 9 cases with loss of MSH6 alone, 6 of 9 were non-MSI-H, thus partly accounting for the decreased specificity with the 4-antibody panel. In conclusion, our results suggest that IHC is useful in detecting MSI in endometrial carcinoma. Although IHC has a lower sensitivity with more apparent staining inadequacies in detecting MSI in endometrial carcinoma than it does in colorectal carcinoma, its use in endometrial carcinoma may be an important adjunct when screening for hereditary cases. In the future, as prognostic and therapeutic implications of MSI phenotype become better defined, it may be reasonable to perform IHC for mismatch repair proteins in large numbers of endometrial carcinomas.