Dogs sniffed out bowel cancer in more than nine out of 10 cases.
A Labrador retriever has sniffed out bowel cancer in breath and stool samples during a study in Japan.
The research, in the journal Gut, showed the dog was able to identify early stages of the disease.
It has already been suggested that dogs can use their noses to detect skin, bladder, lung, ovarian and breast cancers.
Cancer Research UK said it would be extremely difficult to use dogs for routine cancer testing.
The biology of a tumour is thought to include a distinct smell and a series of studies have used dogs to try to detect it.
The researchers at Kyushu University used Marine, an eight-year-old black Labrador.
She was asked to pick from five samples, one of which was from a cancer patient and four from healthy people.
In the breath tests she picked out the cancer sample 33 out of 36 times.
She was even more successful with the stool samples, finding 37 out of 38 cancers.
Even early bowel cancers were detected, which is notoriously difficult.
The NHS screening programme tests for small amounts of blood in faeces, but the researchers believe it picks up only one in 10 early cases.
One in 20 people in the UK develop bowel cancer during their lifetime and more than 16,000 die each year.
Dr Hideto Sonoda, from Kyushu University, said: “It may be difficult to introduce canine scent judgement into clinical practice owing to the expense and time required for the dog trainer and dog education.
“Scent ability and concentration vary between dogs and also within the same dog on different days.
Some early research on developing an “electronic dog’s nose” has taken place, which shows the potential for a cancer breath test.
Dr Sonoda told the BBC: “The specific cancer scent indeed exists, but the chemical compounds are not clear. Only the dog knows the true answer.”
“It is therefore necessary to identify the cancer specific volatile organic compounds [smells] detected by dogs and to develop an early cancer detection sensor that can be substituted for canine scent judgement.
“To complete the sensor useful in clinical practice as a new diagnostic method is still expected to take some time.”
Nell Barrie, science information officer at Cancer Research UK, said: “Although some dogs seem to be able to smell cancer in certain situations, we’re still a long way from understanding exactly what they are detecting and this small study in one dog doesn’t give us any new clues.
“It would be extremely difficult to use dogs as part of routine testing for cancer, and that’s why further research in this area is concentrating on finding out more about the molecules given out by tumours, to see if they could be detected in other ways.”
Mark Flannagan, chief executive of Beating Bowel Cancer, said: “This study looks interesting but it is for the scientists to verify whether these findings could lead to future developments for screening.
“The clear message is that screening saves lives and we encourage everyone eligible to take part in the existing NHS bowel cancer screening programme.”