The airline said it solved the technology problems by Tuesday evening. However, passengers are still reporting long lines at some airports.
Some of the airline's key computer systems were down for more than two hours on Tuesday. And its website was not working.
United says it will not charge the usual change fees for passengers on affected flights who want to cancel or rebook their tickets. It apologized for the disruption.
United Continental Holdings Inc. has been struggling with computer issues off and on after combining some of its major computer systems in March.
A United spokeswoman says the cause of the latest problems is not yet known.