The aims of this study were firstly, to investigate the expression of E‐cadherin complex proteins in ovarian carcinoma cells in serous effusions and in primary and metastatic lesions; and secondly to study the value of these four proteins and calretinin, a mesothelial marker, in the differential diagnosis of ovarian carcinoma cells from reactive mesothelial cells in effusions. Sixty‐seven malignant effusions and 97 corresponding primary (n=36) and metastatic (n=61) lesions were immunohistochemically stained for E‐cadherin and α‐, β‐, and γ‐catenin. Staining extent and intensity were scored. Effusion specimens were additionally analysed for calretinin immunoreactivity. Membrane immunoreactivity for E‐cadherin and α‐, β‐, and γ‐catenin was detected on carcinoma cells in the majority of the effusions, but rarely on reactive mesothelial cells (p<0.001 for all markers). Calretinin immunoreactivity was confined to mesothelial cells (p<0.001). An association was seen between E‐cadherin and α‐catenin expression, in both effusions and solid tumours, and for β‐catenin in solid tumours (range p<0.001 to p=0.014). Up‐regulation of all four cadherin complex proteins was seen in carcinoma cells in effusions, when compared with corresponding primary tumours (range p<0.001 to p=0.028). As with effusions, metastatic lesions showed up‐regulation of α‐, β‐, and γ‐catenin when compared with primary carcinomas (p=0.002–0.015). Carcinoma cells in effusions showed in addition elevated levels of E‐cadherin when compared with metastatic lesions (p<0.001). Staining results in effusions showed no association with effusion site, tumour type or histological grade. Immunoblotting on 29 malignant effusions confirmed the presence of all four proteins in the majority of samples and co‐precipitation of E‐cadherin and β‐catenin was seen in ten specimens examined. E‐cadherin complex proteins are widely expressed in ovarian carcinoma cells. Together with calretinin, they form a powerful battery of markers for the cytological diagnosis of carcinoma cells in effusions. The up‐regulation of E‐cadherin complex proteins in serous effusions and metastatic lesions may mark an early metastatic phenotype and possibly mediates survival of tumour cells at these sites through the inhibition of apoptosis. Copyright © 2000 John Wiley & Sons, Ltd.