Acute kidney failure is when the kidneys suddenly lose the ability to filter waste products from the blood. When this happens, unsafe levels of toxins can build up, and the body’s chemical makeup can get out of balance. It develops rapidly within a few hours or a few days, and is most common in those already in the hospital. It can be fatal and needs thorough treatment, but may be reversible. If you're in generally good health, you may recover normal or nearly normal kidney function.