What causes blood in urine?

Hematuria. Most commonly bladder infections. Gross blood in urine needs to be evaluated by doctor. Other causes could be bladder cancer, passing a kidney stone, interstitial cystitis (inflamation of urinary bladder), prostatitis (in males). Many other cause. Please, see physician.
Bleeding. Hematuria is blood in the urine. It is caused from bleeding somewhere in the genitourinary (gu) tract. That could be in the kidneys (eg kidney cancer or a kidney stone), in the ureters (cancer or stone), bladder (cancer, infection), prostate, or urethra. A urologist can help track down the source and determine what to do.