There are physiologic factors and pathologic factors for high urea in renal function. Physiologic factors are caused by prolonged water fasting. Pathologic factors, except for gastrointestinal bleeding, are mainly caused by acute renal failure and chronic renal failure. Urea is an indicator of renal function, and elevated urea may indicate renal failure. Among them, the main ones that can lead to acute renal failure are acute tubular necrosis and acute progressive glomerulonephritis. The former is mostly caused by nephrotoxic drugs, such as gentamicin, etimicin, ibuprofen, etc., while the latter is mostly caused by glomerulonephritis or connective tissue disease, such as systemic lupus erythematosus, systemic vasculitis, and so on. Chronic renal failure can also cause elevated urea nitrogen. Diseases that can cause chronic renal failure include diabetic nephropathy, uric acid nephropathy, hypertensive nephropathy, purpura nephritis, chronic glomerulonephritis and so on. Acute renal failure and chronic renal failure have different prognosis. Therefore, after the discovery of elevated urea, attention should be paid to find out the cause, be alert to pathological causes, and if it is caused by renal disease, pay attention to distinguish which form of renal failure.