The present immunohistochemical study was carried out on ten cases of ovarian tumours diagnosed with usual staining as undifferentiated ovarian carcinomas. A panel of antibodies was chosen in order to confirm the epithelial origin of these tumours and to eliminate some possible ovarian metastasis. The cytokeratin AE1/AE3, EMA and BerEP4 positivity supported their epithelial origin even if thirty per cent of the cases co-expressed cytokeratin and vimentin. Even if there is not a specific marker to confirm the ovarian origin of the tumours, the CA125 and CEA positivity suggested that these carcinomas might represent a serous or a mucinous dedifferentiation. Associated with these antibodies, the calretinin, CK7 and CK20 stainings allowed the separation of these tumours from peritoneal mesotheliomas with ovarian extension and from ovarian metastasis originating in the gastrointestinal tract. The evaluation of the PCNA labelling index confirms the high degree of cell proliferation and the aggressiveness of these tumours.