Mumford & Sons has canceled its headlining performance at Bonnaroo.
That's because their bassist, Ted Dwane, was recently treated for a blood clot on his brain.
Dwane is recovering from the surgery and is not ready to play Saturday.
They've already postponed three other shows this week.
There's no word on what act will replace Mumford & Sons in the headlining slot in front of 80,000 fans.