Alliance Data Systems Corp said it expected minimal impact on its financial performance and outlook over the "foreseeable future," following the intrusion into the email system at its marketing services unit Epsilon.
The affected clients represent about 2 percent of Epsilon's total client base, Alliance Data said in a statement.
However, the company said the greatest risk to Epsilon and Alliance Data is the potential loss of clients.
"The company's number one priority over the near and long-term will be to ensure that Epsilon's clients regain complete trust in the company's operations," Alliance Data said.
Epsilon is working with federal authorities and other outside forensics experts to investigate the matter and to ensure that any additional security safeguards needed will be implemented.
Last week, Epsilon said its email system had been breached, and email addresses and customer names of some of its clients' customers had been exposed.