Overstock.com, the Utah-based online discount retailer, said CEO and Chairman Patrick Byrne will be taking a personal leave of absence for medical reasons.
Jonathan Johnson, the company's president, will serve as acting CEO.
Byrne, who will remain chairman of the board, in recent years has undergone a number of procedures for heart problems that may have stemmed from an earlier episode of cancer, the company said.
"We've worked hard this past year and delivered good results; our team is executing exceptionally well, putting points on the board. We have set our company plan for 2013 and I have great confidence in my colleagues to execute it," Byrne said in a statement.
Johnson has been with the company for more than 10 years, and has served as president since July 2008. He also serves as corporate secretary and was general counsel.
"My colleagues and I will continue to execute on our plan," Johnson said in the same statement, "and we look forward to the time when Patrick returns to the helm."
Overstock.com shares closed at $12.79 on Tuesday, down 5 cents for the day, and edged back up 2 cents on Wednesday.