Headers have been confirmed as a cause of concussion among footballers, contrary to earlier findings.
A US study found that soccer players who headed the ball a lot were three times more likely to suffer concussion symptoms than those who did not.
Until now ball heading has generally been regarded as harmless, with collisions between players blamed for most cases of concussion.
"These results show that heading the ball is indeed related to concussion symptoms, which is contrary to a recent study that suggested that collisions were responsible for most concussions," said lead researcher Dr Michael Lipton, from Albert Einstein College of Medicine in New York.
"The findings raise concerns about the long-term effects from heading the ball, and more research is needed."
The scientists looked at adult amateur footballers who played for at least six months of the year in the New York city area.
Participants completed questionnaires asking them about their playing frequency, unintentional head impacts, and ball heading.
They were also questioned about impact effects such as pain, dizziness, feeling dazed, and having to stop playing or need medical attention. A very severe impact was defined as losing consciousness.
Of 222 players, men recalled an average of 44 headers in two weeks and women 27.
One or more unintentional head impacts were reported by 37 per cent of men and 43 per cent of women.
Among those who had suffered head impacts, a fifth had moderate to severe symptoms.
Players who had headed the ball the largest number of times were three times more likely to have symptoms than those who headed the ball the least.
The results appear in the latest online issue of the journal Neurology.
