Blood in the semen -- also known as hematospermia -- can be a very alarming symptom. However, the good news is that bloody semen is almost never a sign of a serious underlying medical problem.
Possible sources for blood in the semen include the following:
Urinary tract infections;
Sexually transmitted diseases such as gonorrhea or chlamydia;
Benign prostate hypertrophy;
Surgical procedures such as prostate biopsy or bladder catheterization;
Trauma to the testicles or prostate;
Cancer of the prostate, bladder or reproductive organs.
In most cases, bleeding presumably follows damage to small blood vessels in the urinary tract or prostate gland. This may come from minor trauma or irritation, or growth of normal blood vessels. Small amounts of blood may linger in the semen for weeks, but almost all cases resolve on their own. While on the list of possible sources, it's very rare for bleeding to come from a serious cause such as cancer. This is particularly true for men under the age of 40.
Young men who have a single episode of bleeding that goes away on its own generally need no evaluation other than an examination by your doctor and a urine specimen. A more thorough work-up may be worthwhile in older men, or in men who have persistent or recurring episodes of bleeding. Such a work-up might include cystoscopy, an ultrasound of the bladder and prostate, or a prostate biopsy. However, most of these evaluations will be entirely normal. In fact, even after extensive testing, it's unusual for a specific cause of the bleeding to be identified.