American health insurer Centene Corp says it has lost 950,000 sensitive customer records stored on six hard drives.
The drives hold customers' name and address, date of birth, Social Security numbers, and health information.
Centene Corp boss Michael Neidorff says the company does not know if the information has been compromised.
"Centene takes the privacy and security of our members' information seriously," Neidorff says.
"While we don't believe this information has been used inappropriately, out of abundance of caution and in transparency, we are disclosing an ongoing search for the hard drives.
"The drives were a part of a data project using laboratory results to improve the health outcomes of our members."
Customers receiving "laboratory services" from 2009 to 2015 are affected.
The company says financial information is not included in the lost database.
Affected customers will be handed free credit and healthcare monitoring should someone find the drives and cash in on its contents.
One of the biggest known data breaches from lost devices affected Sutherland Healthcare Solutions in 2014 which lost eight laptops after a February break-in. Some 400,000 records containing customer medical information were lost.
About 774,000 customer records were found on a hard drive in possession of a former Variable Annuity Life Insurance staffer, but it was not thought to have been misused. ®
An earlier version of this story stated 95,000 records had been lost. The true figure is 950,000, as the company has officially announced.