Adult cyclists in Jersey will not be forced to wear a helmet, but a new law will require children to do so.
In a close vote, politicians rejected 25 to 24 the law for adults, but agreed to make them compulsory for under-18s.
Deputy Andrew Green, whose son was left with a brain injury after falling off his bike, had made an emotional plea in the States to bring in the law.
But Deputy Daniel Wimberley opposed the plans, claiming evidence showed wearing a cycle helmet can make injuries worse.
Deputy Green had asked the States to make a helmet a legal requirement either for children or for all cyclists in public places.
He said he had been a passionate supporter of protective headwear for cyclists since his son "had a massive brain injury as a result of a minor head injury" when he was nine.
Nothing could change what had happened to his son, he told the assembly.
"But we can help to reduce the risks for others, and avoid years of emotional and financial costs for individuals and their families."
Deputy Wimberley argued helmets could, in some cases, make injuries worse.
He also believed making helmets compulsory would discourage people in the island from cycling.
"I do believe that this proposal is put forward by a well-intentioned lobby group.
"But they are proposing a law that would affect half of islanders, effectively criminalising them on a scientific basis that is so weak."
Many politicians argued whether it was the States' place to compel people to wear helmets.
Deputy Phil Rondel said the emphasis should be on teaching people the importance of wearing a helmet, rather than forcing them to.
Deputy Sean Power said: " I wear a helmet 99% of the time, but I don't want to be forced to wear a helmet 100% of the time."
Politicians agreed 32 votes to 16 to make it law for under-18s to wear a cycle helmet.