Two people in their 20s died during the Time Warp festival on Friday and three others died in an ambulance or at a hospital.
Officials say most of those still hospitalized are in comas.
Dr. Alberto Crescenti heads the emergency medical system for the Argentine capital. He says doctors are trying to determine what sort of drugs caused the deaths.
The city government said Saturday it has closed the center where the concert took place.
The Time Warp festival was being held in Argentina on Friday and Saturday.
The Buenos Aires’ emergency service was called after it became clear that private ambulances attending the event were unable to cope.
“We couldn’t stay inside, we couldn’t breathe,” one of the participants told local TV channel TN. “It was too hot and there were too many people.”
Time Warp, launched in Germany in 1994, was in Buenos Aires for the third year in a row.
Among the DJs performing there were Chris Liebing, Barem and Deep Mariano.