Emergency officials called five ambulances to the airport about 11 p.m. after a group of people at a gate about to board a Scandanavian Airlines flight reported feeling ill, some with nausea.
Paramedics took six people to Our Lady of the Resurrection Medical Center, and one person to Advocate Lutheran General Hospital in Park Ridge. Later, two additional people were also transported to area hospitals, fire media said. Seven were treated and released from Resurrection.
The passengers were part of a tour group at O'Hare on the way back to Copenhagen, Denmark. The group was on a bus tour before arriving at the airport.
The approximately 40 people that did not get sick were kept at a hotel as a precaution and the group, which had been on a 10-day bus tour, did not leave on the flight. The cause of the illness is under investigation.
"You really need to get ahold of everybody and take a look at how many people were involved. You take a look at how many people were involved in the group, and you take the group out, isolate them and see if they developed any symptoms," said Dr. Darin Harnisch, Resurrection Medical Center.
The Sun-Times Media Wire contributed to this report.