Nearly all older men will experience lower urinary tract symptoms associated with benign prostatic hyperplasia (BPH), the etiology of which is not well understood. We have generated Stk11 CKO mice by conditional deletion of the liver kinase B1 (LKB1) tumor suppressor gene, Stk11 (serine threonine kinase 11), in the fetal Müllerian duct mesenchyme (MDM), the caudal remnant of which is thought to be assimilated by the urogenital sinus primordial mesenchyme in males during fetal development. We show that MDM cells contribute to the postnatal stromal cells at the dorsal aspect of the prostatic urethra by lineage tracing. The Stk11 CKO mice develop prostatic hyperplasia with bladder outlet obstruction, most likely because of stromal expansion. The stromal areas from prostates of Stk11 CKO mice, with or without significant expansion, were estrogen receptor positive, which is consistent with both MD mesenchyme-derived cells and the purported importance of estrogen receptors in BPH development and/or progression. In some cases, stromal hyperplasia was admixed with epithelial metaplasia, sometimes with keratin pearls, consistent with squamous cell carcinomas. Mice with conditional deletion of both Stk11 and Pten developed similar features as the Stk11 CKO mice, but at a highly accelerated rate, often within the first few months after birth. Western blot analyses showed that the loss of LKB1 and phosphatase and tensin homolog deleted on chromosome 10 (PTEN) induces activation of the phospho-5′ adenosine monophosphateactivated protein kinase and phospho-AKT serine/threonine kinase 1 signaling pathways, as well as increased total and active β-catenin. These results suggest that activation of these signaling pathways can induce hyperplasia of the MD stroma, which could play a significant role in the etiology of human BPH.B enign prostatic hypertrophy/hyperplasia (BPH) is one of the most commonly observed proliferative diseases in older men and is characterized by hyperplasia of stromal and epithelial cells of the prostate gland (1). Urinary obstruction resulting from the anatomical enlargement of the prostate in the periurethral region is one of the clinical hallmarks of BPH and the primary indication for medical intervention (2). Several studies investigating the pathophysiology of BPH indicate that the bioavailability of androgens, race, obesity, and other risk factors can contribute to progression of the disease, but age is the most prognostic factor, with estimates of up to 90% incidence in men in their ninth decade (2, 3). However, the precise mechanisms driving BPH have not been completely elucidated. The evidence does suggest that dysregulated stromal cell proliferation is a major component in symptomatic disease (1). Also, BPH appears to develop most often in the transition zone and in the periurethral region, both of which are near the base of the verumontanum (4), a remnant of the fused ends of the caudal Müllerian ducts (MDs). The tendency of BPH to develop in this anatomically distinct region of the prostate sugg...