Judging from the hundreds of videos posted on you-tube, helmet boxing is a favorite among teens, particularly boys as young as 13-years-old.
The game consists of two teens punching each other in the head, over and over until someone is knocked out or gives up.
However, experts say those repeated blows to the head are especially dangerous and could cause damage to a teen's motor skills and speech.
While teens do use helmets, they don't always work.
"The screws that fasten the face mask to the helmet can become loose. They can break. The face mask itself can break which can cause injury to the eyes, nose, to the mouth," said Dr. Andrew Rokito of New York University Sports Medicine.
Doctors say you should look for the warning signs, including cuts and bruises, constant headaches and an unexplained drop in schoolwork
Unfortunately, most parents do not know that helmet boxing is happening in their backyards and basements until it is too late.