Immunohistochemical staining was carried out on a spectrum of normal, hyperplastic and malignant endometrial curettings, for proliferating cell nuclear antigen--PCNA (using the monoclonal antibody PC10) and for abnormally stabilized p53 (using the polyclonal antibody CM-1). The mean proportion of glandular epithelial cells showing PCNA immunoreactivity was significantly lower in atypical hyperplasia/intra-endometrial adenocarcinoma than in invasive adenocarcinoma, but the degree of overlap between the cases was such that this was not considered to be of diagnostic value. p53 immunoreactivity was detected in 47% of invasive adenocarcinomas and in a much smaller proportion of endometria showing simple hyperplasia and atypical hyperplasia, but staining was only focal in the last two conditions. The majority of p53-positive invasive adenocarcinomas had a large proportion of glandular epithelial cells expressing PCNA, but a significant number of p53-negative cases also had a high PC10 index. This suggests that, in endometrial neoplasia, there is not a simple relationship between abnormally stabilized p53 and PCNA expression.