Patients' names, genders, birthdates, home addresses, phone numbers, account numbers, insurance information and reasons for admission may have been exposed.
"At this time, there is no reason to believe the information was used in a malicious manner or one that would impact quality of patient care," the hospital said in a statement. "Long Beach Memorial Medical Center will continue to thoroughly review and strengthen its procedures to ensure it maintains the highest level of patient privacy possible and take all necessary steps to safeguard personal information."
The nature of the breach isn't clear -- a Long Beach Police Department spokeswoman told the Long Beach Press-Telegram that the investigation is open and no arrest has been made.
Potentially affected patients, all of whom were seen between September 2012 and June 2013, are being offered one year of free credit monitoring services. Patients with questions are advised to call (877) 283-6545.