The overall outcome of gallbladder carcinoma has not been favorable because of frequent recurrence at distant sites after surgery. A high-level expression of MUC1 recognized by a monoclonal antibody (mAb), MY.1E12, is correlated with poor survival in several types of carcinomas. There is a need to find new prognostic parameters that can give further insights into tumor aggressiveness of the disease. Immunohistochemistry was performed to determine the expression level of mAb MY.1E12-reactive-MUC1 (MY.1E12-MUC1) in 79 cases of gallbladder carcinoma of different depths of invasion and to determine the correlation of the expression level with clinicopathological findings. The cellular distribution of MY.1E12-MUC1 was heterogeneous among carcinomas of different depths of invasion. Immunohistochemical localization was classified into apical, cytoplasmic and stromal types based on the predominant cellular distribution. In 35 cases of pT2 carcinoma in which curative resections had been performed, the localization was apical type in 54%, cytoplasmic type in 66%, and stromal type in 56% of the cases at the deepest invading sites in the subserosal layer. Postsurgical recurrences at distant sites were seen in 11 of 18 cases of pT2 carcinoma (61%) that had more than or equal to 10% of the cancerous epithelia showing stromal localization of MY.1E12-MUC1 at the deepest invading sites (stromal group) and in 3 of 17 cases (18%) that had less than 10% of the cancerous epithelia showing stromal localization (nonstromal group). The postsurgical 5-year survival rate was significantly poorer in the former (54%) than in the latter (79%; P = 0.049). In 38 cases of pT3 and pT4 carcinomas, the frequency of synchronous distant organ metastasis at the time of diagnosis was significantly higher in cases in the stromal group (61%) than in cases in the nonstromal group (20%). Moreover, in 73 cases of pT2, pT3 and pT4 carcinomas, the expression rate of abnormal localization of E-cadherin was significantly higher in the stromal group (63%) than in the nonstromal group (30%). The MUC1 mRNA levels revealed by in situ hybridization would not be a determinant important for the stromal localization. The stromal localization of MY.1E12-MUC1 may be a phenotype serving as a unique biological feature associated with the tumor aggressiveness of gallbladder carcinoma, such as the tendency to form distant organ metastasis.