A high fat diet increases the risk of the most common form of breast cancer by a fifth, a study has found.
Heavy consumption of saturated fat had a bigger impact, raising the risk of hormone-sensitive breast cancer by 28%.
Fatty diets also made it more likely for a woman to develop breast cancer marked by a defective HER2 gene.
A majority of breast cancers are driven by the female hormone oestrogen, and of these most are also sensitive to the hormone progesterone.
Scientists who studied data on 10,000 patients found an association between fat consumption and breast cancer in these women.
The same link was not seen in women whose cancers were neither hormone-sensitive nor HER2-positive.
The findings, from the Epic breast cancer study involving more than 300,000 women in 10 European countries, are reported in the Journal Of The National Cancer Institute.
The authors, led by Dr Sabina Sieri from the National Tumour Institute in Milan, Italy, wrote: "To conclude, the results of this prospective study on a large heterogeneous population of European women indicate that a high-fat diet increases BC risk and, most conspicuously, that high saturated fat intake increases risk of receptor-positive disease, suggesting saturated fat involvement in the etiology (causes) of receptor-positive BC."