Benign prostatic hyperplasia (BPH) refers to proliferation of smooth muscle and epithelial cells within the transition zone of the prostate. Half of men over 40 develop histologic BPH. About half of men with BPH develop an enlarged prostate gland, called benign prostatic enlargement; among these, about half develop some degree of bladder outlet obstruction. Bladder outlet obstruction and changes in smooth muscle tone and resistance may result in lower urinary tract symptoms, including storage disturbances (such as daytime urinary urgency, frequency, and nocturia) and voiding disturbances (such as urinary hesitancy, weak urinary stream, straining to void, and prolonged voiding).