Chronic fatigue could be linked to sinusitis, scientists have suggested.
Chronic fatigue patients constantly battle tiredness
Researchers in America say many patients with unexplained chronic fatigue also have sinusitis - infected or inflamed sinuses.
The scientists who carried out the study say discovering the link could lead to a treatment for fatigue and pain and help patients who have been unable to receive a diagnosis for their illness.
Researchers in the internal medicine department at Georgetown University Medical Center interviewed almost 300 patients.
Around 22% - 65 - were found to have unexplained chronic fatigue in 22%, unexplained chronic pain in 11%, and both in 9%.
Fifteen out of the 65 patients in Chester's study met criteria for chronic fatigue syndrome (CFS), otherwise known as ME (myalgic encephalomyelitis) .
Sinus symptoms were nine times more common in patients with unexplained chronic fatigue than patients who did not have the condition.
The symptoms were six times more common in patients with unexplained chronic pain.
Researchers also found sinus symptoms were more common in patients with unexplained fatigue than in patients with fatigue explained by a mental or physical illness.
A previous study carried out at Harvard University showed sinusitis patients had worse fatigue and pain scores than another group with congestive heart failure, lung disease, or back pain who were 20 years older.
Professor Alexander Chester, of Georgetown University Medical Center, who led the study, said: "Chronic fatigue is a condition that frustrates both doctors and their patients since treatments directed at just the symptoms without knowing the cause are typically ineffective.
"While sinusitis will not be the answer for everyone who comes to an internist with unexplained fatigue or pain, this study does suggest that it should be considered as part of a patient's medical evaluation."
He added: "We clearly need to do more research to see if sinus treatments alleviate fatigue and pain.
"This study does, however, offer hope for possible help in the future."
Professor Anthony Pinching, medical advisor to the charity Action for M.E., told BBC News Online: "This is an interesting report and may provide some useful clues for both clinicians and researchers.
"It is in line with anecdotal observations. What is not immediately clear is what the association signifies.
"In principle, it could be that one condition increases the risk of the other, but it could go in either direction, or even both, or whether they are both consequences of another different trigger."
He added: "It would be interesting to investigate whether infection or allergic mechanisms, or both, are involved in these sinus symptoms, as this will help us to understand the mechanisms underlying such associations."
The research is published in the journal Archives of Internal Medicine.