The airline told the Federal Aviation Administration that the communication with dispatch was down.
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United issued the following statement Tuesday afternoon:
"Earlier today a software update caused a widespread slowdown in United's technology systems. We briefly held aircraft at their origin airports and resumed normal operations around 12:45 p.m. CT. Our teams are working to get customers to their destinations as soon as possible."
The ground stop was lifted after just overran hour. Many passengers said they were not affected.
By late afternoon Tuesday on the East Coast, United had canceled only seven flights, well below its average of about 16 per day over the busy Labor Day weekend, according to figures from tracking service FlightAware.
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However, more than 350 United flights were delayed - 13% of the carrier's schedule, far more than rivals American, Delta and Southwest - on a day that many holiday vacationers were expected to fly home.
Shares of Chicago-based United Airlines Holdings Inc. fell on news of the ground stop and closed down 2.5%.
The Associated Press contributed to this report.