P27 expression was examined on paraffin-embedded specimens in proliferative, secretory, hyperplastic and neoplastic human endometrium by immunohistochemistry. The results of p27 immunoreactivity in endometrial carcinomas were compared with clinicopathological indicators as well as with p53 expression. Thirty-eight cases of endometrial carcinoma, 30 normal functional (15 proliferative, 15 secretory), 24 hyperplastic endometrium (12 without atypia, 12 with atypia) specimens were studied by using monoclonal p27 and p53 antibodies. The streptavidin-biotin-peroxidase detection system was used and the intensity and the distribution of immunoreactivity was evaluated semiquantitatively. p27 expression was present both in the proliferative and secretory phases; the expression being stronger in the secretory period. In complex hyperplasia with atypia, p27 expression was even higher and it was significantly reduced in the endometrial carcinoma group (p<0.05). No significant correlation was found between p27 expression and any of the clinicopathologic prognostic parameters (p>0.05). Nuclear p53 expression was detected in 13 (34.2%) patients with endometrial carcinoma and was higher in non-endometrioid carcinomas and in tumors with increasing FIGO grade (p<0.05). High expression of p53 was not found to be a significant prognostic indicator of survival (p>0.05). No p53 expression was detected in the endometria with proliferation, secretion or hyperplasia either simple without atypia or complex with atypia. Surprisingly, tumors with absent/low p27 expression showed absent/low p53 expression. Our data suggest that p27 is necessary to control the proliferation of endometrium and its loss of expression seems to play a role in some aspects of endometrial carcinogenesis.