The objective of the present study was to analyze the expression of the proliferation marker, Ki-67, and the anti-apoptotic protein, bcl-2, in various endometrial lesions. Ki-67 and bcl-2 expressions were studied in 194 specimens of endometrial hyperplasia, polyps, carcinomas, and cyclic endometrium from a defined geographic area. Results were statistically analyzed with respect to marker expression, localization to the stromal or glandular component, and intraglandular topography. The lowest glandular Ki-67 expression was seen in secretory endometrium, in polyps, and in atypical hyperplasia. The Ki-67 score was significantly higher and less heterogeneous in endometrial carcinomas than in hyperplasia (p<0.001). Endometrial hyperplasia of all types was characterized by a markedly heterogeneous glandular expression of Ki-67. The glandular expression of bcl-2 was highest in proliferative endometrium and polyps. Bcl-2 expression was significantly lower in adenocarcinomas than in hyperplastic lesions (p=0.002). Ki-67 and bcl-2 expression showed a significant association in proliferative endometrium (p=0.003). Endometrial polyps demonstrated a unique pattern of very low expression of Ki-67 and high bcl-2 expression in both stroma and glands. Our findings indicate that an imbalance between proliferation and apoptosis may be an important factor in the development of different endometrial lesions, benign as well as malignant. The specific finding of inter- and intraglandular Ki-67 heterogeneity may be valuable as an adjunct to morphology in the differential diagnosis of endometrial hyperplasia.