Researchers have identified a link between men who have had vasectomies and the onset of a rare type of dementia.
The scientists from Northwestern University said there was a connection between the sterilisation operation and primary progressive aphasia (PPA), which is a type of dementia affecting people's ability to recall and understand words.
The research, based on 47 men with the condition and 57 without, found that 40 per cent of those with PPA had had vasectomies, while 16 per cent of those without PPA had not undergone the operation.
"That's a huge difference," said Sandra Weintraub, the director of neuropsychology in the Cognitive Neurology and Alzheimer's Disease Center, who led the study.
"It doesn't mean having a vasectomy will give you this disease, but it may be a risk factor to increase your chance of getting it."
Ms Weintraub acknowledged it was a relatively small study, saying further research was needed.
"I don't want to scare anyone away from getting a vasectomy," she concluded. "It's obviously a major birth control alternative. This is just a correlational observation."