Children living near the world's oldest space launch station suffer from high rates of hormonal problems and blood disorders, a journal says.
The rocket fuel, hydrazine, released after take-off, is "nasty and toxic"
The rates of disease close to Baikonur Cosmodrome in Kazakhstan have more than doubled in some cases, Nature reported.
The Siberian study, which has not been published but was leaked to Nature, was rejected by the Russian space agency.
Researchers said unburned hydrazine fuel, which is released during the early stages of take-off, was to blame.
The station, which is rented from Kazakhstan by Russia, was built in the 1950s as a missile testing facility but is now one of the world's largest launch stations.
It is the embarkation point for missions to the International Space Station and in 1961 Yuri Gagarin made history by becoming the first man to orbit the earth after taking-off from the station.
Sergey Zykov, from Vector, the State Research Centre of Virology and Biotechnology in Novosibirsk, the capital of Siberia, led the team which looked at the effect of the fuel on 1,000 children in the country's Altai Republic, which lies in the path of fuel contamination from the rockets.
Children from the worst affected areas were up to twice as likely to require medical attention during 1998 to 2000 compared to the records of 330 children from unpolluted areas.
Mr Zykov told Nature he had discussed the problem with officials from Rosaviakosmos, the Russian space agency, but they had a negative attitude to studies conducted outside their agency.
Fabio Caramelli, an engineer at the European Space Research and Technology Centre, said the fuel was "nasty and toxic".
But Valerie Beral, head of the Cancer Research UK Epidemiology Unit in Oxford, said: "It is hard to know what to make of the findings."
She said that some studies of the 1986 Chernobyl nuclear disaster produced findings that could not be replicated but she added the conclusions needed looking at.
Vyacheslav Davidenko, a spokesman for the Russian space agency, told Nature: "The agency monitors the health of local populations and has found no problem with the launches."
He admitted pollution occurred but said regions were compensated.
And he added any ill health was likely to be due to poor living standards in the region.
Franco Bonacina, of the European Space Agency, which uses the station services, said: "It is not something we are responsible for. It is a matter for the Russian space agency."