A file of personal details of about 1-10th of Yahoo! Japan's 200 million members were likely stolen although it didn't include complete information attackers would need to impersonate users, it said.
Nevertheless, Yahoo! Japan said it would recommend members change passwords to block attempts to take over Yahoo! accounts, the BBC reported Monday.
The company said in a statement it detected an attack on its administration system May 16 and immediately cut off access to it while it investigated the intrusion.
Although the investigation found strong evidence the file had been downloaded, the file didn't contain passwords or information that could be used to reset a password or confirm an identity, Yahoo! Japan said.
The company said it had tightened security measures following the attack.