The significance of DNA mismatch repair (MMR) deficiency or microsatellite instability (MSI) in ampullary carcinomas remains to be defined. This study evaluated the MMR status in 54 consecutive ampullary adenocarcinomas by immunohistochemical and morphologic studies. All tumors were moderately (n = 49) or poorly (n = 5) differentiated, with 7 mucinous and 1 signet-ring cell type. Tumor-infiltrating lymphocytes (TILs) were noted in 36 tumors. Loss of MMR protein by immunohistochemical analysis was identified in 3 (6%), 2 lost MSH6, and 1 lost MLH1/PMS2. One MSH6- case had 3 metachronous colorectal cancers. Five TILs per 10 high-power fields predicted immunohistochemical abnormality in 2 of 3 tumors with a specificity of 80% (41/51); however, none of the 5 tumors that had the highest TIL counts (20-62/10 high-power fields) showed abnormal immunohistochemical results. Thus, MMR deficiency occurs in ampullary carcinoma but appears less frequent than in colorectal carcinoma (CRC). Typical MSI-high histologic features of CRC, such as increased TIL counts, seem to have similar yet subtly different implications in ampullary carcinoma.