5T4 oncofetal antigen is defined by a monoclonal antibody raised against human placental trophoblast, and recognizes a 72 kD glycoprotein expressed in many different carcinomas but detected only at low levels in some normal epithelia. Analysis of the patterns of expression of 5T4 oncofetal antigen in colorectal carcinomas has indicated a significant association between the presence of the antigen in tumor cells and metastatic spread. The 5T4 antigen expression of 72 epithelial ovarian carcinomas has been investigated by immunohistochemistry; 71% of the carcinomas demonstrated positive 5T4 immunoreactivity in adenocarcinoma cells and/or associated stromal tissue. In order to assess any relationship to prognosis, the 5T4 phenotypes were analyzed with respect to various clinicopathologic features of the tumors and the clinical outcome of the patients assessed by survival and disease-free interval. There was a significant correlation between 5T4 expression and more advanced stage of disease (FIGO stages III and IV) (P< 0.001) and with poorly differentiated tumors (P= 0.036) compared to well or moderately differentiated tumors. Patients with tumors expressing 5T4 were less likely to respond well to adjuvant therapy (P= 0.030) and had a significantly worse outlook in terms of survival (P= 0.033) and disease-free interval (P= 0.033). This significance was not demonstrated as acting independently of FIGO stage and tumor differentiation.