Women who drink lots of coffee may cut their risk of developing one type of breast cancer, according to a new study, but experts are urging caution.
The Karolinska Institute findings are based on nearly 6,000 women and suggest drinking more than five cups a day halves a woman's risk.
But cancer experts say the evidence is not proof enough and women should instead focus on leading a healthy lifestyle to cut their cancer odds.
They say the findings need confirming.
The Swedish researchers are now doing more studies to dig deeper into the coffee question.
Their initial findings suggest that coffee cuts the risk of an aggressive form of breast cancer called oestrogen-receptor negative breast cancer.
Around one in four women diagnosed with breast cancer will have oestrogen-receptor negative breast cancer, which is often resilient to drug treatment and requires intensive chemotherapy.
In the study, published in BioMed Central's open access journal Breast Cancer Research, post-menopausal women with and without breast cancer were asked about lifestyle factors and coffee consumption.
The findings revealed the association between higher coffee intake and lower risk of breast cancer.
Research Professor Per Hall said: "We were surprised. But the result was clear. When we looked more closely the more aggressive form of cancer - the oestrogen-receptor negative - was reduced by over half, meaning half as many women who drank lots of coffee were diagnosed with this cancer."
But he said what is not clear is why coffee might be having this effect.
"We just don't know what might be behind this association. There are so many different compounds in coffee that it could be any one of them that could be having an effect."
The other problem is the study rests on the women reliably recalling how much coffee they drink, and there is no record of what type of coffee they consumed - espressos, cappuccinos or decaf?
Until more work is done, Professor Hall says he would not advise women to up their coffee consumption, especially since too much coffee can have side effects.
Yinka Ebo of Cancer Research UK, said: "This study does not provide firm evidence that drinking lots of coffee can help reduce the risk of breast cancer as it relied on people with cancer remembering how much coffee they drank years ago.
"Previous research on coffee consumption and breast cancer risk has produced mixed results, and the authors of this new study acknowledge that further work will be needed to confirm the findings."
He said there was good evidence, however, that women who are physically active, consume little or no alcohol and keep a healthy weight after the menopause are at lower risk of breast cancer.