Having recent prostate surgery or a prostate biopsy can cause blood in semen for several weeks after the procedure.

Most often, no cause can be found for blood in semen. Infection might be a cause. But infection is likely to have other symptoms. These may include pain while urinating or urinating more often.

A lot of blood in semen or blood that keeps coming back might be a warning sign for conditions such as cancer. But this is rare.

Possible causes of blood in semen:

1. A lot of sexual activity or masturbation.
2. Blood vessel malformation, a tangle of blood vessels that disrupts blood flow.
3. Conditions that cause the urinary or reproductive organs to become inflamed.
4. Infections of the urinary or reproductive organs from germs or fungus.
5. Not having sex for a long time.
6. Radiation therapy to the pelvis.
7. Recent urological procedures, such as a bladder scope, prostate biopsy or vasectomy.
8. Trauma to the pelvis or genitals.
9. Side effects of medicines that thin the blood, such as warfarin.