Prostatitis causes substantial morbidity to men, through associated urinary symptoms, sexual dysfunction, and pelvic pain; however, 90% to 95% of cases have an unknown etiology. Inflammation is associated with the development of carcinoma, and, therefore, it is imperative to identify and study the causes of prostatitis to improve our understanding of this disease and its role in prostate cancer. As estrogens cause prostatic inflammation, here we characterize the murine prostatic phenotype induced by elevated endogenous estrogens due to aromatase overexpression (AROM؉). Early-life development of the AROM؉ prostate was normal; however, progressive changes culminated in chronic inflammation and pre-malignancy. The AROM؉ prostate was smaller at puberty compared with wild-type controls. Mast cell numbers were significantly increased at puberty and preceded chronic inflammation, which emerged by 40 weeks of age and was characterized by increased mast cell, macrophage, neutrophil, and T-lymphocyte numbers. The expression of key inflammatory mediators was also significantly altered, and premalignant prostatic intraepithelial neoplasia lesions emerged by 52 weeks of age. Taken together, these data link estrogens to prostatitis and premalignancy in the prostate, further implicating a role for estrogen in prostate cancer. These data also establish the AROM؉ mouse as a novel, non-bacterial model for the study of prostatitis. Prostatitis, an exceedingly common condition in the male population worldwide, has an incidence of ϳ9% and a prevalence of ϳ14%, and, unlike benign prostatic hyperplasia and prostate cancer (PCa), which are predominantly diseases of older men, prostatitis afflicts men of all ages. It is also the most common outpatient condition seen in men under 50 years of age and accounts for more clinical visits than either PCa and/or benign prostatic hyperplasia.1,2 At present our knowledge of prostatitis is lacking, with 90% to 95% of cases having an unknown etiology. This represents a significant problem, particularly as prostatitis has also been implicated in the development of PCa.3-5 Given the prevalence of prostatitis and this putative link to PCa, it is vitally important to identify the unknown causes of prostatitis.Several causes of prostatitis have been postulated, including hormonal variation or exposure, infection due to sexually transmitted disease, 6 dietary factors, and physical trauma from urine reflux. 7 However, of particular interest is an apparent link between estrogen and prostatic inflammation, 8 which has emerged mainly from the administration of pharmacological levels of exogenous estrogen to rodents.9 Additional data obtained from further rodent studies show that the prostate gland is particularly sensitive to estrogenic exposure during development in fetal and neonatal life; transient estrogen exposure before puberty results in inflammation later in life, well after the exposure has occurred.10,11 Several decades of research from various laboratories, including our own, has demonstrated that...