The high court on Wednesday overturned a lower court decision to certify as a class customers who say the company's monopoly in parts of the Philadelphia area allowed it to raise prices unfairly.
Justice Antonin Scalia said in a 5-4 decision the customers need to be able to prove more of their case early in the process, including whether damages can be calculated for the entire group. The Comcast subscribers had argued that doing so would lead to even more limits on class actions.
Justices Stephen Breyer and Ruth Bader Ginsburg wrote the dissent jointly for themselves, Sonia Sotomayor and Elena Kagan.
In an unusual move, Breyer and Ginsburg read parts aloud in the courtroom.