The differentiation between reactive mesothelial and carcinoma cells in serous effusion cytology can be a diagnostic challenge based on morphology alone. The expression of some cell adhesion molecules may be helpful in the differential diagnosis. This study evaluated the usefulness of E-cadherin Immunocytochemistry for discrimination of carcinoma cells from reactive mesothelial cells. Alcohol fixed, paraffin embedded cell blocks taken from 42 reactive and 102 malignant serous effusions with histologically confirmed diagnoses were immunostained with monoclonal antibody to E-cadherin by LSAB method. E-cadherin expression was identified in only 2 benign reactive serous effusions(5%) whereas 91 malignant serous effusions(89%) expressed E-cadherin The differences in immunostaining for E-cadherin between reactive and malignant serous effusions were statistically significant(p < 0.001). The sensitivity and specificity of the E-cadherin immunostaining for carcinoma cells were 89% and 95%, respectively. In conclusion, E-cadherin is a useful diagnostic adjunct for differentiation between reactive mesothelial and carcinoma cells in serous effusions.