Officials said someone at the Paradiso Festival was peddling a drug promoted as ecstasy, but was actually "Molly," a powerful form of ecstasy sometimes mixed with other drugs.
Many of the people who took the drug overdosed, and local hospitals were overwhelmed with patients.
Sarina Fahrner, the chief nursing officer at Quincy Valley Medical Center, said the hospital treated more than 100 overdose patients from the concert, all of whom took the drug Molly.
"When you have cocaine and LSD and meth all mixed together, it just does terrible things to your body," she said. "Your heart doesn't tolerate it very well, your breathing does not tolerate it very well."
She said the overdose patients ranged in age from 18 to 26 years old.
Fahrner said Quincy Valley Medical Center is the closest hospital to the Gorge Amphitheatre, so they often treat people who overdose on drugs during concerts. But this weekend was the worst they've seen as far as the number of patients, she said.
She said several of the patients were critically ill and had to be sent to other hospitals for treatment in intensive-care units.
Shawn Wertman, the supervisor at Central Washington Hospital in Wenatchee, said his hospital received the most seriously-ill patients, including a young man who died early Sunday of an apparent overdose.
The name of the man who died has not been released.
Officials with the Gorge Amphitheatre and Live Nation Entertainment could not be reached for comment on Sunday.