We examined immunohistochemical PTEN protein expression in endometrial samples obtained from perimenopausal women with uterine bleeding in order to diagnose early endometrial neoplasia. Paraffin sections of endometrial samples (biopsies, endometrium resection, hysterectomy) were reviewed and tested for immunocytochemical PTEN expression by epithelial cells. Immunohistochemical studies were performed with the Ventana Benchmark device and the immunoperoxidase technique with monoclonal anti-PTEN (6H2.1/Cascade Biosciences). We studied 45 samples of proliferative endometria, 42 samples of atypical hyperplasia (neoplasia), and 55 samples of endometrial carcinomas (30 endometrioid, 5 serous papillary, 10 clear cell, 5 adenosquamous, and 5 MMMT). A strong positive PTEN immunoreaction was observed in all 45 normal proliferative endometria, 2/30 endometrioid carcinomas, and 23/25 non endometrioid carcinomas. In contrast, PTEN immunoexpression was negative or weak in 40/42 atypical hyperplasia and 28/30 endometrioid carcinomas. Negative or weak PTEN expression correlated with endometrial neoplasia (atypical proliferation) (p < 0.001). Thus (i) PTEN immunoexpression in endometrial paraffin sections is a new diagnostic tool, distinguishing PTEN-positive cyclic abnormal monoclonal proliferative endometrium (anovulatory, non atypical, simple and complex hyperplasia) from PTEN-negative (or decreased) endometrial neoplasia, and especially early endometrial neoplasia (atypical simple and complex hyperplasia, in situ carcinoma, superficial carcinoma with focal invasion). (ii) Given the lack of reproducibility of histological identification of atypical (precancerous invasive) and non atypical (precancerous non invasive) endometrial hyperplasia, the new criteria (PTEN-negative crowded glands, gland/stroma ratio > 55%, nuclear pleomorphism, in areas > 1 mm) recently proposed to diagnose early endometrial neoplasia appear to be clinically relevant.