The IAAF says its has been hacked by the 'Fancy Bears' group and fears that athletes' therapeutic use exemption (TUE) data has been compromised.
Athletics' world governing body was notified last month but it is unaware whether information was stolen.
Athletes who have applied for a TUE were contacted on Monday.
IAAF president Lord Coe said: "Our first priority is to the athletes who have provided information they believed would be secure and confidential."
His statement added: "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 revealed that "the presence of unauthorised remote access to the IAAF network by the attackers was noted on 21 February".
Russian group 'Fancy Bears' first hacked the World Anti-Doping agency (Wada) database on 13 September last year and began revealing athletes' confidential details and information regarding TUEs - which let athletes take prohibited substances if there is a medical need.
Sir Bradley Wiggins has also faced scrutiny following the leak of his medical records in September 2016.