It can be scary to see blood in urine, also called hematuria. In many cases, the cause is harmless. But blood in urine also can be a sign of a serious illness. If you can see the blood, it's called gross hematuria. Blood that can't be seen with the naked eye is called microscopic hematuria. It's such a small amount that it can be seen only under a microscope when a lab tests the urine. Either way, it's important to figure out the reason for the bleeding