Sudden back pain and blood in urine in men may be caused by renal cysts, acute pyelonephritis, ureteral stones and other factors. 1. Renal cysts: when the diameter of renal cysts is small, the clinical symptoms are usually not obvious, but when the size of renal cysts is large and the renal area has been excessively squeezed, it may lead to rupture of the cysts and bleeding, which will cause low back pain and the symptoms of hematuria. 2. Acute pyelonephritis: usually caused by upward infection or bloodstream infection, the onset of the disease is more acute, there may be symptoms of urinary pain and blood in urine, accompanied by urinary difficulty, urinary frequency, urinary urgency and other symptoms, there may also be systemic symptoms, such as fever, fatigue. 3. Ureteral stones: usually secondary to kidney stones, when the stone is stuck in the ureter, it will cause irritation and damage to the ureter, which may cause low back pain as well as hematuria, and low back pain is more intense. Men with sudden back pain and blood in urine should seek medical attention for detailed examination and treatment to avoid other serious complications.