Airlines are paying more for fuel, labor and maintenance, drawing scrutiny from investors who fear the industry’s rising costs threaten margins during a record stretch of profitability.

Expenses at the nine largest airlines rose 8.1% in the first nine months of 2017 compared with the prior-year period, according to the Airlines for America trade group, while revenue rose 3.8%. The run-up in expenses is well above the overall U.S. inflation rate of 2.2%.

The...