Many children diagnosed with attention deficit disorder may just simply be the youngest members of their class, Taiwanese researchers suggest.
Looking at data on 400,000 children, they found August-born children were almost twice as likely to have ADHD as those born the previous September.
The team said many cases may be down to teachers comparing the behaviour of the least and the most mature children.
ADHD is an inability to control activity levels and sustain attention.
It is a behavioural condition which affects up to 7% or 400,000 British children and impairs their ability to learn.
The study of Taiwanese children, in the Journal of Pediatrics, found 2.8% of the pre-school and primary school boys born in September are diagnosed with the condition compared with 4.5% of those born the following August. For girls the incidence increased from 0.7% to 1.2%.
In Taiwan, as in England, the birthday cut-off for the school year is August 31.
The study said: "Worldwide, the number of children and adolescents being diagnosed with ADHD or receiving a prescription for ADHD, has significantly increased.
"Evidence shows that relative age, which may be a proxy of neuro-cognitive ability, may increase the likelihood of ADHD diagnosis and medication."
Lead author Dr Mu-Hong Chen, of the Department of Psychology at Taipei Veterans General Hospital in Taiwan, said: "Our findings emphasize the importance of considering the age of a child within a grade (year-group) when diagnosing ADHD and prescribing medication to treat ADHD."
In England, the difference in development at primary school level between autumn-born pupils and their younger, summer born peers is well known.
Those born later in the year tend to be more likely to be diagnosed with special educational needs.
Consultant psychiatrist and chairman of the ADHD Foundation, Dr Kuben Naidoo, said: "While the study findings indicate an increase in the rate of diagnosis and treatment for ADHD in this Taiwanese population, the authors acknowledge significant limitations including absence of key information such as family history and environmental factors.
"The study therefore highlights the importance of ensuring the assessment for ADHD is rigorous and relies on a variety of sources of information that support the clinician in deciding whether the diagnosis is met.
"In the UK setting the assessment and diagnosis of ADHD across the lifespan is robust and relies on information gathered from a number of sources, including the family and school.
"This is then coupled with information obtained from a clinical interview by a specialist paediatrician or psychiatrist.
"The option to treat with medication is not taken lightly and consideration is also given to psychological strategies to support the individual. This decision on the type of treatment would be influenced by the degree of impairment experienced by the individual."