We examined the immunohistochemical localization of p53, a tumor suppressor gene, in thirty-three specimens of uterine endometrial carcinoma which included one case of carcinosarcoma, eleven specimens of endometrial hyperplasia and ten specimens of normal endometrium. We also analyzed the association between the immunolocalization of p53 protein and the clinical and pathological parameters of endometrial concern. We also determined whether p53 mRNA is overexpressed in these specimens by in situ hybridization and simultaneous immunohistochemistry and in situ hybridization. Immunohistochemistry was performed using the monoclonal antibodies pAbDO-7 and pAb1801, and polyclonal antibody pAbCM-1. Immunoreactive p53 was observed in the nuclei of the cancer cells in 15/33 (45%) by pAbDO-7, 11/26 (42%) by pAb1801, and 16/33 (48%) by pAbCM-1. No p53 immunoactivity could be detected in either hyperplasia or normal endometrium except for a case in which the endometrium was in the secretory phase. There was no significant relationship between p53 immunostaining as determined by pAbDO-7, and age, clinical stage, histological grade or depth of myometrial invasion. We employed the 35S-labeled antisense single stranded synthetic oligonucleotide probe, ON102, to perform in situ hybridization and simultaneous immunohistochemistry and in situ hybridization. In every case of endometrial carcinoma studied, no significant accumulation of the p53 hybridization signal was observed in carcinoma cells. This indicated that overexpression of p53, as observed by immunohistochemical staining, is not due to an increase in the steady-state level of p53 mRNA in carcinoma cells. These results suggest that immunostaining of p53 is associated with the malignant phenotype, but does not correlate with the biological behavior of human endometrial carcinoma.