Organisation's president says they will look to remedy the situation
Athletics' world governing body says hackers, thought to be from Russia, have broken into their database of confidential medical records.
The International Association of Athletics Federations (IAAF) president Sebastien Coe has apologised to athletes.
His organisation says the Fancy Bears group has taken information about exemptions for using otherwise banned substances.
British sportspeople Bradley Wiggins and Mo Farah were among those targeted last year in a hack on the World Anti-Doping Agency.
"Our first priority is to the athletes who have provided the IAAF with information that they believed would be secure and confidential," said Coe in a statement.
"They have our sincerest apologies and our total commitment to continue to do everything in our power to remedy the situation and work with the world’s best organisations to create as safe an environment as we can."
The IAAF statement explained that the cyber attack was "detected during a proactive investigation carried out by cyber incident response (CIR) firm Context Information Security, who were contacted by IAAF at the beginning of January to undertake a technical investigation across IAAF systems".