What causes benign prostatic hyperplasia?

As men age, they often develop benign prostatic hyperplasia (BPH). The prostate gland can grow from the size of a walnut to about the size of a lemon. It's not clear why the prostate grows like this, but it's believed certain male hormones such as dihydrotestosterone tend to act more strongly on the prostate gland later in life. Because the prostate is located just below the bladder, when it becomes larger it can place pressure on the urethra, the tube that carries urine from the bladder through the penis and out of the body. This may lead to a variety of urinary symptoms.