Blood in urine is clinically known as hematuria, and the possible causes are: acute glomerulonephritis, IgA nephropathy, urinary stones, urinary malignant tumors and so on. 1. Acute glomerulonephritis: Acute glomerulonephritis is a kind of renal parenchymal disease caused by infection-induced immune response of the body, which is common among children and adolescents, and the patients often have acute onset of the disease, with the main manifestations of hematuria, edema, and elevated blood pressure. 2. IgA nephropathy: IgA nephropathy is the most common primary glomerular disease in the world, most common in young and middle-aged men, IgA nephropathy has insidious onset, mostly manifested as asymptomatic hematuria, or accompanied by non-specific symptoms such as fatigue, muscle pains, and general malaise, etc. The prognosis of IgA nephropathy varies a lot, part of the patient’s renal function can be maintained for a long period of time, but some of the patients can experience rapid renal failure or even develop end-stage renal disease. The prognosis of IgA nephropathy varies greatly. 3. urinary stones: urinary stones rubbing the uroepithelium or embedded in the ureteral stenosis can damage the uroepithelium and cause bleeding and manifest as hematuria. 4. Malignant tumor of urinary system: various malignant tumors of urinary system can rupture and bleed causing hematuria, such as renal cancer, ureteral cancer, bladder cancer, prostate cancer and so on. Older people with painless hematuria should be highly suspected of malignant tumor of urinary system. In addition to the above diseases, urinary system tuberculosis, urinary tract infection, trauma, etc. can cause hematuria. It is recommended that patients go to the hospital as soon as possible to find out the cause of hematuria and get targeted treatment.