The usefulness of E-cadherin immunostaining as a marker of malignancy in the body fluids was investigated in the present study. Thirty-three histologically proven cases of cell blocks from the pleural, peritoneal, and pericardial fluids were studied by immunocytochemistry for E-cadherin antibody using LSAB method. These cases were cytologically diagnosed as adenocarcinoma (25 cases) and atypical cells (8 cases). Tumor cells showed strong positive membranous staining for E-cadherin antibody in 21 out of 25 cases (84%) of adenocarcinoma. E-cadherin staining was not found in 6 of 8 cases of suspicious maligancy. The sensitivity and specificity were 84% and 75%, respectively. Reactive mesothelial cells and inflammatory cells scattered were all negative. In conclusion, E-cadherin is an useful adjunctive marker to distinguish reactive mesothelial cells from the carcinoma cells in the body fluids.