Travelers -- or almost-travelers -- were really unhappy with airlines last year. 

In fact, the U.S. Department of Transportation (DOT) received 102,550 complaints about airlines and travel agencies from January 2020 to December 2020, according to the department’s most recent Air Travel Consumer Report, released Thursday.

The DOT said the number of complaints is the "highest number on record" and up 568.4% from the number of complaints it received in 2019, which was just 15,342, according to a press release from the department. 

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Of those 102,550 complaints, most were about refunds. 

According to the report, there were 89,518 complaints about refunds, which makes up 87.29% of all of last year’s complaints. 

Most of the complaints the DOT received were about refunds, according to the department’s report. (iStock)

Most of the complaints the DOT received were about refunds, according to the department’s report. (iStock)

Of those, 29,687 complaints were about U.S.-based airlines’ refunds, while 45,856 complaints were about non-U.S.-based airlines’ refunds. Another 13,742 complaints were made about refunds from travel agents and 78 complaints were made about refunds from tour operators.

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"The Department’s Office of Aviation Consumer Protection has been in direct communication with the largest U.S. and foreign air carriers and other carriers that received a large number of refund complaints to ensure compliance with the refund requirements," the DOT said in a statement. "Many passengers who had initially been denied refunds have received the required refunds. The Department will take enforcement action against noncompliant airlines and ticket agents as necessary."

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Following refunds, the DOT received 4,449 complaints about reservations, ticketing and boarding, 3,300 complaints about fares, 1,499 complaints about flight problems, and 1,483 complaints about customer service, according to the report.

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As far as overall complaints, the DOT’s report found that United Airlines had the highest number of complaints among U.S.-based airlines, with 11,274 complaints. 

However, Frontier Airlines had the highest rate of complaints, with 49.3 complaints per 100,000 passenger boardings.