Sheldon Marks, MD: Blood in the semen is, almost always, nothing at all. It is a symptom of a small blood vessel probably breaking, often during sex,sometimes when men are bearing down with heavy lifting or straining, sometimes with a difficult bowel movement if they're constipated, sometimes for no reason at all.So bright red blood in the semen, speckled, pink, brown, that happens. And unless it's serious, meaning it's heavy and prolonged, we just ignore it.It's is not something to be worried about. It's very scary. Especially if you don't know what to expect and suddenly you see blood in the semen, then you panic.But it's rarely a problem. Now if you're an older guy, it could be a sign of prostate cancer or a kind of urethra or bladder cancer. But I've only seen that once in 20 years.It's almost always nothing. In fact some of the major textbooks don't even address the matter of seminal blood.