People with Huntington's disease, a debilitating brain condition, appear have a "protection" from cancer, according to a study in Sweden.
Nearly 40 years of medical records showed patients with Huntington's had half the normal expected risk of developing tumours.
Researchers, writing in The Lancet Oncology, said the reason was unclear.
Cancer Research UK said the findings presented another avenue to explore in tackling cancer.
Academics at Lund University analysed Swedish hospital data from 1969 to 2008. They found 1,510 patients with Huntington's disease.
During the study period, 91 of those patients subsequently developed cancer. The authors said that was 53% lower than the levels expected for the general population.
Huntington's is one of a group of illnesses called "polyglutamine diseases". Data from other polyglutamine diseases also showed lower levels of cancer.
The authors said: "We found that the incidence of cancer was significantly lower among patients with polyglutamine diseases than in the general population.
"The mechanisms behind the protective effects against cancer are unclear and further research is warranted."
Dr Jianguang Ji, from the Center for Primary Health Care Research at Lund University, told the BBC: "Clarification of the mechanism underlying the link between polyglutamine diseases and cancer in the future could lead to the development of new treatment options for cancer."
Eleanor Barrie, senior science information officer at Cancer Research UK, said: "These are interesting results. It's not clear how the genetic changes that cause Huntington's and other similar diseases could protect against cancer, and research in the lab will help to find out more.
"Scientists at Cancer Research UK and around the world are probing the genetic faults that contribute to cancer in their quest to beat the disease, and this is another potential avenue to explore."