Endometrial polyps are a frequent cause of abnormal uterine bleeding, but their pathogenesis is poorly understood. This study was undertaken to investigate if endometrial polyps result from localized overexpression of estrogen receptors (ERs) or reduced expression of progesterone receptors (PRs). Fourteen cases of endometrial polyps, in which normal cycling endometrium was also present on the same slide, were immunostained for ERs and PRs. Percentages of positive cells in glands and stroma for each receptor were subjectively assessed to the nearest 5%. The intensity of staining was recorded on a scale from 1+ to 4+. The level and intensity of staining in polyps were compared with the staining in normal endometrium. Fewer stromal cells in polyps expressed ERs and PRs compared with cycling endometrium (% ER = 55.9 +/- 25.8 vs. 74.3 +/- 25.8, p = 0.03; % PR = 56.1 +/- 28.2 vs. 87.5 +/- 10.1, p = 0.002). Stroma in polyps also had significantly reduced intensity of staining for PRs, but not for ERs (intensity PR = 2.7 +/- 1.4 vs. 3.5 +/- 0.7, p = 0.015; intensity ER = 2.1 +/- 0.7 vs. 2.4 +/- 0.8, p = 0.45). There were no significant differences in expression of ERs and PRs in the endometrial glands in endometrial polyps compared with normal endometrium (% ER = 75.4 +/- 32.5 vs. 70.7 +/- 39.2. p = 0.25; % PR = 79.6 +/- 32.8 vs. 80.4 +/- 34.4, p = 0.8; intensity ER = 2.7 +/- 0.9 vs. 2.4 +/- 1, p = 0.15; intensity PR = 2.9 +/- 1.4 vs. 3.4 +/- 0.7, p = 0.15). We conclude that endometrial polyps may result from a decrease in ER and PR expression in stromal cells. Because of these receptor-negative stromal cells, endometrial polyps may be relatively insensitive to cyclic hormonal changes.