The airline put out a message on its Twitter page saying the problem started about 7 p.m. The company is working to resolve the matter.
In addition to departures, the computer outage is also affecting airport processing and reservations.
The Federal Aviation Administration says as a result of the problem, United passengers are unable to check in.
Passenger Susan Suchor of Palos Heights told ABC7 her United flight had been sitting at the gate at Boston's Logan Airport for nearly an hour Friday night. Because of computer problems, the airline said it couldn't calculate how much fuel the plane needed, Suchor said.
For more information, visit United's website at www.united.com.