"We looked at blood-mercury levels in children who had autism and children who did not have autism," said lead author Irva Hertz-Picciotto, a professor of environmental and occupational health.
"The bottom line is that blood-mercury levels in both populations were essentially the same. However, this analysis did not address a causal role, because we measured mercury after the diagnosis was made," she added.
Earlier research has shown that mercury can adversely affect development of the nervous system.
The research, published in the journal "Environmental Health Perspectives," is the largest investigation to date on mercury levels in the blood of autistic children.
The study was done as part of the California-based Childhood Autism Risks from Genetics and the Environment (CHARGE) Study, of which Hertz-Picciotto is lead investigator.
CHARGE is a comprehensive epidemiological investigation that seeks to identify factors associated with autism and discover clues to its origins.
Children who took part were aged between 24 and 60 months and diagnosed with autism as well as children with other developmental disorders. Children who developing typically were used as controls.
The study probed sources of mercury in the participants' environments, such as fish consumption, personal-care products (such as nasal sprays or earwax removal products, which may contain mercury) and the types of vaccinations they received, researchers said.
"The study also examined whether children who have dental fillings made of the silver-colored mercury-based amalgam and who grind their teeth or chew gum had higher blood-mercury levels," they added.
"In fact, those children who both chew gum and have amalgams did have higher blood-mercury levels.
"But the consumption of fish -- such as tuna and other ocean fish and freshwater fish -- was far and away the biggest and most significant predictor of blood-mercury levels," they stressed.
The study was carried out on 452 children: 249 were diagnosed as autistic, 143 were deemed to be developing normally and 60 showed retarded development such as Down Syndrome.
"Just as autism is complex, with great variation in severity and presentation, it is highly likely that its causes will be found to be equally complex. It's time to abandon the idea that a single 'smoking gun' will emerge to explain why so many children are developing autism," said Hertz-Picciotto.
"The evidence to date suggests that, without taking account of both genetic susceptibility and environmental factors, the story will remain incomplete," she added.
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