Older people with Alzheimer's are less likely than those free of the disease to have cancer -- and cancer patients have a reduced risk of Alzheimer's disease, the findings show.
"Since the number of cases of both Alzheimer's disease and cancer increase exponentially as people age, understanding the mechanisms behind this relationship may help us better develop new treatments for both diseases," study leader Dr Massimo Musicco, from the Italian National Research Council in Milan, said.
Dr Musicco's team monitored 204,468 people aged 60 and over in northern Italy over a period of six years.
During that time, 21,451 of the participants developed cancer and 2,832 were diagnosed with Alzheimer's.
But just 161 had both cancer and Alzheimer's disease - a much lower figure than expected.
Compared with the general population, the risk of cancer was halved in people with Alzheimer's disease and the risk of Alzheimer's was reduced by 35% in those with cancer.
The findings appear in the online issue of the journal Neurology.