Up to 31 athletes from six sports could be banned from competing at the Rio Games, Olympic chiefs have said.
The announcement comes after the International Olympic Committee (IOC) retested 454 selected doping samples from the 2008 Games in Beijing.
The IOC said the retests were conducted using the very latest scientific analysis methods.
It also revealed it is awaiting the results of 250 retests from the 2012 Olympics in London.
More than 4,500 tests were carried out at the Beijing Games in 2008 - but just nine athletes were caught cheating.
The IOC said the retests were focused on athletes who could potentially take part in Rio.
It added 12 affected national Olympic associations would be informed in the coming days.
However, the IOC said it would not be revealing the names of athletes who had returned adverse findings until B-samples had been tested and individuals informed, foreign media reports.