Immunohistochemical analysis of curettage material from a placental site trophoblastic tumor (PSTT) revealed a high expression of p53 gene products, of epidermal growth factor receptor (EGF-R) and of Ki-67 (MIB-1) proliferation associated antigen. bcl-2 was not expressed. These results show that in PSTT inactivation/dysregulation of p53 and upregulation of EGF-R and MiB-1 occurs, indicating that these factors are probably involved in tumor genesis and propagation of PSTT. The prognostic significance of the molecular genetic data, however, remains to be established.