A clinical trial has been launched to see if a breath test could detect the presence of cancer.
Researchers want to find out if signals of different cancer types can be picked up in patterns of breath molecules.
The Cancer Research UK team in Cambridge will collect breath samples from 1,500 people, some with cancer.
If the technology is proven, the hope is that breath tests could be used in GP practices to decide if patients need to be referred for more tests.
They could potentially be used alongside blood and urine tests to help doctors detect cancer at an early stage, the researchers said.
But it will be two years before the results of the exploratory trial are known.
GPs' leaders said the research was exciting but they warned patients that breath tests to detect cancer were "unlikely to be commonplace at their GP practice anytime soon".
How does the test work?
Molecules called volatile organic compounds (VOCs) are released when cells in the body carry out biochemical reactions as part of their behaviour.
But if cancer or other conditions are present, the normal behaviour of cells is altered and they appear to produce a different pattern of molecules - and a different signature smell.
The research team is trying to find out if this pattern or odour can be identified in people's breath, using breath biopsy technology.
Their ultimate aim is to work out if different types of cancer produce different patterns - or signatures - which can be detected at an early stage.
What's the potential for the test?
This is the start of the trial so we won't know for several years whether or not the initial results are promising.
The science behind the test itself is not new.
Many researchers around the world have been working on the possibility of breath tests for a number of cancers, including lung, for a number of years.
There are some promising signs that breath tests could detect pre-cancerous symptoms, but it is not yet clear how accurate they are.
Any breath test used on large numbers of patients would have to be sensitive and accurate to avoid misdiagnoses and false positives.
In short, there is a long way to go and much more research needed on more people before a breath test will be appearing in any GP surgeries.
It is possible that dogs could be also used to sniff out the odours given off by cancers, and other diseases like Parkinson's.
Who will take part in the trial?
The trial will start with patients with suspected oesophageal and stomach cancers and then widened to include people with prostate, kidney, bladder, liver and pancreatic cancers in the coming months.
Healthy people will also be included in the trial.
At Addenbrooke's Hospital in Cambridge, participants will be asked to breathe into a face mask for 10 minutes so a sample can be collected.
The samples will then be sent to a laboratory in Cambridge to be analysed.
'Best chance of surviving'
Rebecca Coldrick, 54, was one of the first people to take part in the trial. She has a condition called Barrett's oesophagus and could go on to develop cancer.
"I was very happy to take part in the trial and I want to help with research however I can," she said.
"I think the more research done to monitor conditions like mine and the kinder the detection tests developed, the better."
Prof Rebecca Fitzgerald, lead trial investigator at the Cancer Research UK Cambridge Centre, said: "We urgently need to develop new tools, like this breath test, which could help to detect and diagnose cancer earlier, giving patients the best chance of surviving their disease."
Dr David Crosby, head of early detection research at Cancer Research UK, said breath tests were a technology that had the potential "to revolutionise the way we detect and diagnose cancer in the future".
Cancer Research UK has made research into this area one of its top priorities.