Sega has become the latest computer game giant to fall prey to hackers.
The company sent an email to users of the Sega Pass system, warning that email addresses, dates of birth and encrypted passwords had been obtained.
However it said that no financial information was at risk.
The Sega Pass system was taken offline on Thursday and all users' passwords have been reset.
Customers have been advised to be on the alert for suspicious emails asking for further personal information.
This comes after attacks on Sony and another games company, Codemasters.
In April Sony had data stolen from the 77 million users of the PlayStation network, one of the worst security breaches of its kind.
There was then a second attack when another 24.6 million computer game users might have had their personal details stolen.
Personal information and passwords were taken in the hack on Codemasters earlier this month, but not financial details.
The hackers who claimed responsibility for the Sony attack, LulzSec, have apparently denied the Sega intrusion.