The airline said Tuesday night that it had cancelled 90 flights so far. The grounding covers about one-fifth its fleet.
Southwest spokeswoman Brandy King said that after discovering the missed inspections, the airline immediately notified federal safety regulators and began checking the planes.
A spokesman for the Federal Aviation Administration, Lynn Lunsford, said that the FAA was working with Southwest and Boeing, which manufactured the planes, to evaluate a plan that would let the airline keep flying the planes until the inspections are completed over the next few days.
The missed inspections were first reported by The Wall Street Journal.
King called the missed inspections inadvertent. She said the airline discovered that 128 of its Boeing 737-700 jets already had flown beyond the point at which the backup hydraulic systems were supposed to be inspected. She said safety was the airline's top priority, and it was working to fix the matter quickly.
Dallas-based Southwest is the nation's fourth-biggest airline. It has 665 jets, all of which are some version of the Boeing 737, including nearly 450 of the 737-700. That model seats 137 or 143 passengers, depending on the layout.