Depression in cancer survivors significantly increases the chances of an early death.
Dutch researchers found that cancer survivors who became depressed were twice as likely as other survivors to die prematurely.
The study looked at depression in people one to 10 years after they had cancer.
They analyzed just over 3000 survivors and data collected over several large studies.
Patients had survived endometrial cancer, colorectal cancer, lymphoma or multiple myeloma.
Those who showed signs of depression two to three years after their cancer were found to be twice as likely to die prematurely.
It is unclear if they died because the cancer recurred or from another illness.
It is also not clear why depression and cancer are linked.
"The next step is to investigate the possible mechanisms that might explain the association between depressive symptoms and death from cancer," said study author Floortje Mols, in a statement.
"We also need to better understand whether treatments for depressive symptoms in cancer patients have life-prolonging effects."
The study was published in the Journal of Cancer Survivorship.