Southwest Airlines reported a 0.3% decline in traffic in December, in comparison with the same month last year.
Capacity increased by 1.8%, as passenger numbers grew marginally by 0.4% to 8.93 million.
Load factor for the month fell 1.7 percentage points to 78.4%.
For the 12 months ended December, traffic 6.4%, while capacity grew 4.9% on the year.
Passenger numbers increased 2.4% to 110 million, as load factor increased 1.1 percentage points to 80.8%.