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.
"All these measures are a powerful strike against the cheats we do not allow to win," IOC president Thomas Bach said.
"They show once again that dopers have no place to hide. We keep samples for 10 years so that the cheats know that they can never rest.
"By stopping so many doped athletes from participating in Rio, we are showing once more our determination to protect the integrity of the Olympic competition."
The IOC said the re-tests 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.