The five young men and women in the car were on their way home from music festival Emmaboda, according to the Aftonbladet newspaper.
The accident occurred at 5.30am on Sunday morning, as their car drove off the road and into a ditch. All five were taken to the Högland hospital in Eksjö, one of them with severe injuries.
“They are all young, and one of them, a woman, has been badly injured,” said Jönköping police spokesman Tommy Thomgren to the TT news agency.
The car has been confiscated by the police, who will be conducting a forensic investigation of the vehicle, according to national broadcaster Sveriges Radio (SR).
It's unclear why their car drove off the road, but the police say there is no indication of any crime having been committed.
“We've started an investigation and will be speaking with these people as soon as possible,” said Thomgren.
The police have yet to reveal what part of Sweden the five young people come from.
Two young men have been treated and released from the hospital, but the three others are still hospitalized.