A spokesperson at LAX says the impact there is minimal, though passengers are being checked in manually. They can also expect long lines.
United experienced a network outage for more than two hours, and flights to its hubs in San Francisco, Houston and Newark were on hold. The problem appears to be resolved.
United has struggled with computer issues since combining some of its major systems last spring.
The airline says it will not charge the usual change fees for passengers on affected flights who want to cancel or rebook their tickets, and the company apologized for the disruption.
A spokeswoman says the cause of the latest problems is not yet known.
The Associated Press contributed to this report.