Three dozen people were transported to the hospital and as many as 50 others were treated or evaluated at the scene during a an electronic dance music show in Boston featuring Swedish disc jockey Avicii.
No one appeared to have life-threatening injuries, Boston EMS Deputy Superintendent Mike Bosse said, blaming alcohol and drug use for the problems at the Wednesday night show at the TD Garden.
All of those transported and treated appeared to be between 16 and 25 years old, he said.
Some concert-goers blamed oppressive heat inside the arena.
Bosse said he contacted the Boston Police Licensing Division, which cited the TD Garden for allowing lots of intoxicated concertgoers to enter.
Jacqueline Peterson, a spokeswoman for concert promoter Live Nation, said early reports indicate that a majority of those taken to the hospital "were people outside the venue who were too impaired to be allowed inside" for the concert. They were taken for medical evaluation and assistance as a precaution, she said in a statement early Thursday.
Avicii, whose real name is Tim Bergling, called the medical issues "awful news" on his Twitter account. "Its a terrible thing, I rly hope everyone is ok! My thoughts go to those affected & their families."
Two people died over the weekend at a Las Vegas music festival attended by Avicii, one of the biggest names in electronic dance music. Authorities said Monday it will take several weeks to determine what caused the deaths of the Electric Daisy Carnival fans.
Police said nearly 800 people were treated for medical conditions and 25 were hospitalized during the three-night electronic music festival, which organizers said drew about 400,000 people at the Las Vegas Motor Speedway. Festival organizers told police that 134,000 fans attended Friday, Saturday and Sunday night shows featuring pulsing lights, Ferris wheels, seven stages of music and DJs including Avicii, Diplo, Afrojack and Tiesto.