Blood in the urine: Blood that appears in the urine. Also known as hematuria. Gross hematuria refers to blood that is so plentiful in the urine that the blood is visible with just the naked eye. Microhematuria refers to blood in urine that is visible only under a microscope; there is so little blood that it cannot be seen without magnification. Hematuria, whether gross or microscopic, is abnormal and should be further investigated. It may or may not be accompanied by pain. Painful hematuria can be caused by a number of disorders, including infections and stones in the urinary tract. Painless hematuria can also be due to a large number of causes, including cancer.