An ambulance on a 999 call was stolen from a driveway while the crew were attending to a woman in her home.
The vehicle was found abandoned a short time later after it had become stuck in mud on wasteground close to a river.
The woman did not need hospital care in the alert in Bridgend early on Sunday, but the ambulance service said it was "disgusted" by the theft.
South Wales Police said a man wearing red shorts was seen running from the ambulance.
The ambulance thief struck after paramedics were called to treat a woman in the Cefn Glas area of the town at about 0100 GMT.
Nothing was stolen from the vehicle, but it was found to have been damaged in a "minor" accident with another vehicle, believed to be stationary at the time, before being abandoned.
"It could have been the difference between life and death"
The vehicle was checked by a mechanic and senior officers and was returned to service by 0230 GMT.
The ambulance service has launched a joint investigation with South Wales Police.
Rob Jeffrey, production manager for the ambulance service, said the vehicle "may well have ended up in a river" if it had not become bogged down in mud.
He said: "We're dismayed and disgusted.
"Clearly, the people who took the ambulance were not aware of what we were dealing with in the home and any number of calls could have come in in that time.
"It could have been the difference between life and death."
He said if the paramedic crew had been dealing with a life-threatening emergency, a replacement ambulance would have needed to be despatched from Porthcawl.
Police are appealing for anyone with information to call them on 01656 655555 or Crimestoppers on 0800 555 111
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