The number of dementia patients in the UK is predicted to double
Alzheimer's disease is associated with a reduced risk of cancer and vice versa, a study suggests.
US researchers followed 3,020 people aged 65 and above for the study, published in the journal Neurology.
Those who had Alzheimer's at the start of the study were 69% less likely to be admitted to hospital with cancer than those free of the disease at the start.
And those with cancer at the study's start were 43% less likely to develop Alzheimer's than the cancer free.
The researchers followed the subjects for an average of five years to see whether they developed Alzheimer's, and an average of eight years to see whether they developed cancer.
At the start of the study, 164 people (5.4%) already had Alzheimer's disease and 522 people (17.3%) already had a cancer diagnosis.
During the study, 478 people developed dementia and 376 people developed invasive cancer.
The researchers stressed that more work was needed before any firm conclusions could be drawn, and said the findings only seemed fully to apply to white people.
They found no association between cancer and another type of dementia, known as vascular dementia, which is thought to be caused by a lack of blood supply to the brain.
However, patients with this condition died earlier than people with Alzheimer's.
Lead researcher Dr Catherine Roe, of Washington University School of Medicine in St Louis, said this suggested the association between Alzheimer's and cancer was not simply due to people with those conditions dying before they could contract the other ailment.
"Discovering the links between these two conditions may help us better understand both diseases and open up avenues for possible treatments," she said.
"Alzheimer's disease and cancer are both characterised by abnormal, but opposing, cellular behaviour.
"In Alzheimer's disease, excessive cell death occurs, whereas cancer is characterised by excessive cell growth.
"Other scientists have suggested that there are certain molecular pathways that may influence both Alzheimer's disease and cancer."
For instance, one specific enzyme has been shown to target a number of proteins, some of which are believed to stimulate cancer, some to suppress it, and others to be a hallmark of Alzheimer's.
Rebecca Wood, chief executive of the Alzheimer's Research Trust, agreed the study raised hopes of finding new ways to prevent or treat disease.
She said: "This study suggests that there might be a link between cancer and Alzheimer's, but it is much too soon to say for certain whether the two diseases are connected.
"There could be molecular processes involved in both Alzheimer's and cancer, which, with more research, we could identify."
However, Professor Clive Ballard, of the Alzheimer's Society, said the existence of one of the diseases could mask the symptoms of the other and affect diagnosis.
"More research is needed to establish categorically if this link exists."
It is estimated 700,000 people in the UK have dementia, a number forecast to double in a generation.