A stream near Sheffield is being inspected by environment agency officers amid reports that three dogs have died after walking in the area.
The pets' owners said their animals had died suddenly in the last few weeks after being taken for walks on Townend Common next to Stocksbridge Golf Club.
They fear the dogs may have drunk contaminated water from a nearby beck.
An Environment Agency spokesman said a visual inspection was taking place on Friday to assess the need for tests.
Dog owner Julie Horton, of Deepcar, said her seven-year-old Bedlington Terrier, Ted, died last Tuesday, the day after she had taken him for a walk.
"The minute we got back home he became violently sick with breathing difficulties," she said.
"His breathing got worse through the night and eventually he couldn't breathe any longer.
"Within 24 hours he was dead."
She fears that water may have become contaminated by chemicals from a former landfill site, which is now part of the golf course.
Neighbour Julie Smith, who owned a seven-year-old Schnauzer, said: "Our dog was exactly the same - when he got back from the walk he could not breathe, he was sick and he died a terrible death."
She said her sister's dog had also succumbed to the mystery illness last October after walking in the same area.
The Environment Agency has urged any other dog owners whose animals have suffered a similar reaction to report the incidents.
"We are trying to get to the bottom of it to calm people's worries," a spokesman said.
"One of our environment enforcement officers is visiting the site this morning [Friday] to look at the watercourse.
"If the visual inspection finds anything suspicious there will be formal testing of the water."
He said a local vet who treated two of the dogs felt they may have died from a viral infection.