A third of women will suffer a migraine over the course of their life
Women who suffer regular migraines may have the comfort of knowing they face a much lower risk of breast cancer, say US researchers.
The discovery points to the potential importance of hormone levels in both.
The study of 3,412 women suggests a 30% lower risk for people with a history of disabling headaches.
However, the researchers, from the Fred Hutchinson Cancer Research Center in Seattle, warned more work was needed to confirm the link.
While an estimated 30% of women will suffer at least one migraine in their lifetime, for a much smaller group, they are a regular feature.
Scientists have connected the condition, which can feature nausea and visual disturbances alongside severe headaches, with fluctuating levels of hormones.
Being pregnant, or on the contraceptive pill, both of which affect hormone levels, can lead to noticeable changes in the frequency and severity of attacks.
The US researchers are the first to look at whether this might have an effect on the chances of developing breast cancer, which, in two of its most common forms is fuelled by the hormones oestrogen and progesterone.
Their group of women included 1,938 who had been diagnosed with breast cancer and 1,474 who had no history of the disease.
Women were asked to report whether they had ever been diagnosed with migraines by a health professional.
Those with a history of migraines were far less likely to go on to develop breast cancer.
Dr Christopher Li, reporting the results in the journal Cancer Epidemiology, Biomarkers and Prevention, said that the hormone hypothesis appeared the most likely.
A high-oestrogen state, he said, such as that found in pregnancy, could be linked to both a reduction in attacks, and the conditions needed to stimulate breast cancer development.
He said: "While these results need to be interpreted with caution, they point to a possible new factor that may be related to breast cancer risk.
"This gives us a new avenue to explore the biology behind risk reduction."