Flights at Chicago's O'Hare International Airport were experiencing delays of up to 90 minutes after a problem with the runway lighting system.
Four of the seven runways were closed for about 45 minutes on Thursday night because the edge lights were either flickering or completely out, Federal Aviation Administration officials said.
The problem affected four runways on the north side of the airport -- 27L, 27R, 32R and 22R -- and prevented planes from taking off or landing. Aircraft were diverted to the remaining three runways.
Karen Pride, the spokeswoman for Chicago's Department of Aviation, said the problem was corrected shortly after 10 p.m. The FAA reported the outage at 9:19 p.m.
Pride said she didn't know what caused the outage but said officials were investigating.
Travelers were advised to keep in contact with their carriers for the most up-to-date schedule information.