Lung cancer patients who have surgery in the winter are 40% more likely to die of the disease than those operated on in the summer, a US study suggests.
Lung cancer is responsible for nearly a quarter of all cancer deaths
A study of 456 patients found high levels of vitamin D - from sun exposure and food supplements - had a positive impact on the success of surgery.
The Harvard University team said more research was needed and patients should not expect surgery in the summer.
UK experts said it was interesting but warned sun exposure could be dangerous.
Lead researcher Wei Zhou said: "This study in no way suggests that people should try to time their cancer surgeries for a particular season - that would obviously be impossible.
"But if validated it may mean that increasing a patient's use of vitamin D before such surgery could offer a survival benefit."
Researchers studied the treatment of 456 lung cancer patients of which only 10% had had either radiation treatment of chemotherapy.
Looking at the effect of the seasons, the team found patients who had operations in the winter were 40% more likely to die from their cancer than those who had the operation in the summer.
When the joint effect of the season and vitamin D levels were taken into consideration, there was a three-fold better chance of survival, evidence presented to the American Association of Cancer Research showed.
Lung cancer is responsible for 33,600 deaths a year in the UK - nearly a quarter of the total number of cancer mortalities.
The reason for the effect of the vitamin was not clear either, the researchers said.
Dr Kat Arney, science information officer at Cancer Research UK, said more research was needed to see if increasing the levels of vitamin D before surgery would be beneficial.
But she added: "There is no substantial evidence to suggest that excessive sun exposure or vitamin D supplementation can have a protective effect against cancer in healthy people.
"Although vitamin D is made by our bodies in response to sunlight, we actually need relatively little exposure to maintain healthy levels.
"Any extra vitamin D made in this way cannot be stored. Cancer Research UK advises everyone to be 'SunSmart', as prolonged sunbathing, sunbed use and especially sunburn in children can all increase the risk of skin cancer later in life."