Kidney punches are illegal in boxing. A hard blow to a kidney can bruise or cut the organ, or even tear it loose from the blood vessels that supply it. Damage can range from mild pain to blood in the urine to anemia, kidney failure, shock and even death. That's why the rules of boxing, which generally prohibit any punch to the back, pay special attention to the kidney area.
The most commonly used rules for boxing are those drawn up by the International Boxing Association, the sport's world governing body. The association, which goes by its original French initials AIBA, is different from sanctioning bodies such as the WBA, WBC, WBO and IBF. The sanctioning bodies are the organizations that declare champions in professional boxing; the AIBA sets rules for matches, sanctions amateur events and governs international competitions such as the world championships and boxing in the Olympic Games.
Under AIBA rules, any blow that lands on the back of an opponent can be called a foul by the referee. The rules instruct referees to pay especially close attention to punches that land on the back of the neck, known as a "rabbit punch," or on the kidney area -- the lower back, on either side of the spine, just above the pelvis.
The referee has discretion in declaring penalties for a kidney punch. Depending on the severity of the punch and whether the referee believes it was intended to injure, he can issue a caution, a warning or an immediate disqualification. Cautions are delivered on the fly, with the referee simply reminding the fighter to watch where he throws his punches. For a warning, the referee stops the fight to tell the fighter to watch himself. At this point, the judges of the match vote on whether they agree with the warning; if they do, the warned boxer's opponent is awarded 2 extra points for the round. Once the referee has issued a warning for a particular foul, he can no longer issue cautions, only warnings. So if a boxer has been warned about kidney punches, the next such punch will draw a formal warning, with the potential net loss of 2 points. A third warning results in an automatic disqualification, and the fighter's opponent wins the match.
Turning the Back
Punches to the abdomen are perfectly legal. If a boxer is throwing such a punch and his opponent turns his back, the punch might strike the kidney area. If the referee believes the kidney punch was not intentional, he need not call a foul. In fact, if the referee thinks the other fighter deliberately turned his back to the punch, he can call a foul on that fighter instead.