Blood test of lung cancer patients could help predict success chances of chemotherapy treatment.
Patients carrying a newly identified molecule in their blood are less likely to respond to drugs, UK researchers say.
There is more than one type of lung cancer, but the variety under investigation by Liverpool-based researchers, small-cell lung cancer, which accounts for between 15% and 20% of cases, is one of the more difficult to treat.
Even small cell lung cancer comes in different forms, with a version called "neuroendocrine" being the least likely to be treated successfully.
The researchers found that a molecule called SCG3 mRNA was more likely to be found in the blood of people with neuroendocrine small cell cancer.
In theory, if larger studies back this up, it could mean that patients arriving at the clinic could be tested to give doctors an idea of the likely success of therapy - or perhaps to predict when a patient was relapsing before other signs emerged.
It might also make it easier for scientists, when looking at new chemotherapy treatments in trials, to compare their effectiveness in different groups of small cell lung cancer patients.
Dr Judy Coulson, from the University of Liverpool, said: "There are currently no blood-based markers routinely used to monitor patients with this type of lung cancer.
"We found that SCG3 mRNA is an incredibly sensitive marker of these tumours and could be used to detect circulating tumour cells in patients with this disease."
Lung cancer kills more than 30,000 people in the UK every year, and survival rates have not improved alongside those for breast or bowel cancer in recent years.
Cancer Research UK said the discovery could help doctors choose the right kind of treatment for patients.
Lesley Walker, from Cancer Research UK, said: "This discovery is an important step to understanding how to treat lung cancer patients more effectively.
"Lung cancer can be very difficult to treat in its later stages, either because it has spread or because there are too many tumours."