Important features of the origin and evolution of benign prostatic enlargement (BPH) remain unclarified, partly because of imprecision in previous morphologic observations. Precise, quantitative analysis was applied to BPH development in 63 autopsy prostates. BPH nodules originated selectively from a very small region, near a cylindrical urethral sphincter above the verumontanum, and usually on the outer aspect of that sphincter laterally. They arose in a newly described transition zone, in which the unique mingling of prostatic glands with sphincteric stroma may be implicated in BPH pathogenesis. Nodules originate through eccentric duct budding toward a focus, suggesting local stromal inductive effects. BPH evolved through three processes: early diffuse gland growth, small nodule proliferation, and later nodule enlargement. If these are independent processes, BPH etiology may be multifactorial.