A delicate balance in estrogen and progesterone signaling through their cognate receptors is characteristic for the physiologic state of the endometrium, and a shift in receptor isotype expression can be frequently found in human endometrial pathology. In this study, using a transgenic mouse model, we examined the mechanisms whereby alterations in progesterone receptor (PR) isotype expression leads to endometrial pathology. For an experimental model, we used transgenic mice (PR-A transgenics) carrying an imbalance in the native ratio of the two PR isoforms A and B (PR-A and PR-B) through the expression of additional A form and examined their uterine phenotype under different hormonal regimens, using various criteria. Uterine epithelial cell proliferation was augmented in PR-A transgenics and was abolished by PR antagonists. In particular, proliferative response to progesterone, independent of signaling through estrogen, was enhanced. Upon continuous exposure to estradiol and progesterone, the uteri in PR-A transgenics displayed gross enlargement, endometrial hyperplasia including atypical lesions, endometritis and pelvic inflammatory disease. Imbalanced expression of the two isoforms of PR in a transgenic model reveals multiple derangements in the regulation of uterine physiology, resulting in various pathologies including hyperplasias.