From the bench, Roberts said Ginsburg was "indisposed due to illness," but offered no details. He added that she would rely on written briefs and audio recordings to keep up with the cases. A court spokeswoman later said Ginsburg was at home with a stomach bug.
The court was scheduled to hear oral arguments on two cases Wednesday, including a $20 billion racial discrimination case, Comcast Corporation v. National Association of African American-Owned Media.
Ginsburg, 86, attended every court session in her 25-year tenure until she missed a session in January following surgery to remove cancerous nodules from her lung. In August, the court said she was undergoing treatment for a "tumor on her pancreas." She previously was treated for colon and pancreatic cancer.
Ginsburg is regarded as the leader of the court's liberal wing.
The Wednesday session was the court's last this month. Justices will hold a private meeting on Friday, and meet again on Monday, but no arguments are scheduled. Case arguments will resume on Dec. 2.