The brains of elderly patients with hearing loss appear to decline more rapidly than those with full hearing, a US study shows.
Suggested explanations include rewiring of the brain as hearing declines or social isolation caused by not being able to communicate.
The researchers hope that treating hearing loss can slow the onset of cognitive decline and dementia.
The study was published in the journal JAMA Internal Medicine.
A team of researchers at Johns Hopkins University followed 1,984 people in their 70s for six years - all had hearing and mental ability tests at the beginning of the study. There were then follow-up brain tests over the next six years.
Test scores declined as the study progressed, but patients with a hearing loss deteriorated 40% more quickly.
One of the researchers, Dr Frank Lin, said there were two main theories about how the two could be linked.
Becoming socially withdrawn due to hearing loss - such as not going out or struggling with conversation - has been linked to cognitive decline and dementia in the past.
Another idea is "cognitive load". As hearing declines the brain dedicates more resources to interpreting the information it is sent, stealing brain power which would be used for other functions.
Cognitive decline can be an early symptom of dementia.
Dr Lin told the BBC: "The major public health question is if we treat hearing loss can we delay cognitive decline or dementia?
"That's what we all care about, but the answer is we just don't know."
He said people with hearing loss used a hearing aid in only 15% of cases in the US "so it is very undertreated".
Dr Eric Karran, from the charity Alzheimer's Research UK, said the exact connection between hearing loss and cognitive decline was still unknown.
He said: "Potential social isolation caused by hearing impairment is a more likely explanation for this link than there being a shared disease process, although this needs further investigation, this will be an interesting area to study further.
"Many people find their hearing becomes worse as they get older, and age is also the biggest risk factor for dementia.
"Understanding whether the two are directly linked could give important insight into the condition, but more research will be needed to fully answer this question."
Dr Jason Warren, from University College London, researches how dementia affects the way people hear the world around them.
He said brain scans had shown that hearing loss in patients was also down to the way a dying brain tried to handle sound.
"People with Alzheimer's often become 'lost' trying to follow a conversation in a noisy room or over the telephone.
"We hope ultimately to design and target effective interventions to help people manage such symptoms in their everyday lives."