The nation's largest carrier asked the Federal Aviation Administration to halt its flights just before noon Monday until 2 p.m. after a system-wide issue regarding its reservations and check-in system.
"Any American plane sitting on the ground anywhere in the U.S." won't be taking off, said FAA spokesman Lynn Lunsford.
Passengers are paying the price as more than 1,000 flights were canceled from coast to coast. Of them, about 10 had been canceled by 4 p.m. at Los Angeles International Airport.
Travelers were advised to call ahead or check with the airline's website before arriving at LAX.
The timing of the computer outage didn't help matters coming the day after the Boston Marathon bombings.
"It is important to note we see no evidence that today's technical outage is related to the tragic events in Boston," American Airlines said in a statement.
The Associated Press contributed to this report.