Premera Blue Cross said it has been the victim of a "sophisticated" cyberattack, in which its systems and databases were breached.
As many as 11 million people are affected by the breach, the company said, including members and prospective members of its services and associated brands.
According to the company, a slew of personal information of members and applicants was taken, including names, date of birth records, email and postal addresses, phone numbers, Social Security numbers, and bank account information.
Claims and clinical information are also understood to have been taken in the attack.
"We also have no evidence to date that such data has been used inappropriately," the note read.
The Mountlake Terrace, Wash.-based health insurance company serves millions of users across Washington, Oregon, and Alaska, among other states.
The attack on Premera's systems occurred at the same time as the Anthem health insurance breach on January 29, in which around 80 million customer records were stolen. Around 19 million non-customers of Anthem's services were also affected by the hack. The FBI said in late February that it was "close" to identifying the hackers.
Premera warned in late February that it was "processing" Anthem's data to see if its customers were affected by the breach.