Background: polycystic ovary syndrome (PCOS) is a common disorder of women in reproductive age. PCOS, a complex genetic condition, is a heterogeneous syndrome of clinical and/or biochemical androgen excess, ovulatory dysfunction and polycystic ovaries. Metformin has been shown to improve fertility in the ovulatory patients with PCOS, inducing not only a high ovulation and pregnancy rate, but also reducing the incidence of miscarriages. Aim of the Work: this study aimed to evaluate the effects of metformin on hormonal profile and endometrial tissue, including pattern of immunohistochemical expression of androgen receptors (AR), in patients with PCOS. Patients and Methods: 50 women with polycystic ovary syndrome (PCOS) were enrolled in this study. Each patient was submitted to detailed history taking, clinical examination including hair all over the body, body weight, body height and body mass index (BMI), transvaginal sonography, laboratory investigations ( LH, FSH, fasting insulin, free testosterone). Endometrial samples were taken for histopathological evaluation and assessment of androgen receptors. These investigations were done before and after three months of metformin treatment. Results: there was a significant decrease of BMI of the studied cases after metformin administration (P value <0.001). Also, there was a significant decrease of LH level after metformin administration from 8.07 ± 2.48 mIU/ml to 6.85 ± 3.5 mIU/ml and a significant decrease of fasting insulin level from 13.5 ± 5.3 before metformin administration to 10.2 ± 6.9 after administration. As regard to pathological results before metformin treatment, histopathological examination of the endometrial specimens revealed features of early proliferative endometrium in 32 cases, late proliferative in 10 cases, while the remaining 8 cases showed features of simple endometrial hyperplasia. Among the 32 cases with early proliferative endometrium, 30 cases progressed to late proliferative endometrium after treatment with metformin, while the remaining 2 cases showed no morphologic changes. Among the 10 cases with late proliferative endometrium, 3 cases progressed to early secretory phase, 3 cases to mid-secretory and 4 cases progressed to late secretory endometrium indicative of ovulation. Regression of hyperplasia was noted in 3 out of the 8 cases diagnosed as simple endometrial hyperplasia. As regard to immunohistochemistry (IHC), we found that women with PCOS exhibited marked increase in endometrial androgen receptors (AR) expression compared to the normal fertile controls (p<0.001). Significant decrease in AR expression in endometrial tissue after metformin therapy was observed (p<0.003). Conclusion: metformin therapy in patients with PCOS restores menstrual cyclicity, induces ovulation and showing decrease in LH level, fasting insulin, and AR expression in endometrial tissue.