Autopsies will be needed to determine the substance involved, said Alberto Crescenti, head of the Emergency Health Care System, which was summoned to the Time Warp music festival in the early morning hours.
The partygoers were mostly between the ages of 21 and 25, Crescenti said.
"The youths lost consciousness and rapidly went into a coma," he said.
The initial death toll was four, but edged up after one victim originally hospitalized died later today.
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It was the third Time Warp festival in Buenos Aires, attracting some 10,900 attendees, officials said.
The city government said that the venue, a large private events and exhibition center, had been approved to hold the festival and host as many as 13,000 people.
Time Warp festival is a well-known electronic music festival, originating in Germany about two decades ago.
The line-up in Buenos Aires included DJs such as Sven Vath, Jamie Jones, Ricardo Villalobos, Luciano, Maceo Plex and Tale Of Us.