Sony says it has contacted the FBI and taken action to protect its websites after intrusions by a group of hackers.
"We have confirmed that a breach has occurred and have taken action to protect against further intrusion," said Michael Lynton, chairman of Sony Pictures Entertainment.
"We also retained a respected team of experts to conduct the forensic analysis of the attack."
A group calling itself LulzSec posted statements online saying it broke into SonyPictures.com and downloaded unencrypted personal information, including passwords, email addresses and dates of birth from one million user accounts.
The attack was the latest on Tokyo-based Sony, which in the past two months said more than 100 million accounts were compromised after hackers broke into its networks.
Sony, which has only just resumed full operation of the PlayStation Network in the US and Europe after six weeks of suspension, said last month the intrusions will cost about 14 billion yen ($213 million) this fiscal year.
"My biggest concern is whether the expense related to unauthorised accesses will stay within 14 billion yen," said Tsunenori Ohmaki, an analyst at Tachibana Securities in Tokyo.
LulzSec posted customer information online from what appeared to be sweepstakes and loyalty-programme databases, saying: "It's just a matter of taking it ... this is disgraceful and insecure."