A major study appears to provide hard evidence that eating a high-fat diet increases a postmenopausal woman's risk of breast cancer.
Women were asked how often they ate 124 kinds of food
Animal research has found a high fat intake boosts cancer risk, but other studies in humans were inconclusive.
This work, by the National Cancer Institute, asked 188,700 women about their diets, and found a link between breast cancer and eating more fat.
UK breast cancer campaigners said women should eat a healthy diet.
All the women in the US study were aged between 50 and 71 when they became involved in the research.
The US researchers asked the women, who were all postmenopausal, how often they ate 124 different foods, ranging from never to up to six times a day, and about the portion size.
The women were then followed up for an average of four years.
The amount of fat the women took in was measured as a percentage of their total energy intake, ranging from 20% in the fifth which ate least fat to 40% in the fifth that ate most.
Out of all the women surveyed, 3,501 developed invasive breast cancer during the course of the research.
Those who ate the most fat had an 11% higher incidence of breast cancer than those who ate the least.
The increase in risk was similar whether the women were eating saturated, monounsaturated or polyunsaturated fats.
Taking into account family history of breast cancer, smoking, body mass index and alcohol intake did not affect the results.
However, those women with the highest fat intake were more likely to have been taking hormone replacement therapy (HRT) - which has been linked to breast cancer - when they joined the study.
Writing in the Journal of the National Cancer Institute, the authors, led by Dr Annie Thiebaut, said: "In this large cohort of postmenopausal US women, we detected a direct association between dietary fat intake and the risk of invasive breast cancer."
They suggest the reason other studies have sometimes failed to find the link is because they did not have participants who obtained 20% or less of their energy from fat, making it harder to establish the difference.
And they suggest fat may affect breast cancer risk by stimulating hormone production.
The researchers say more work is needed to further understand the risk.
But in an editorial in the same journal, Stephanie Smith-Warner and Meir Stampfer, of the Harvard School of Public Health in Boston, said it was more important to control the amount of body fat a woman had, rather than her fat intake in order to prevent breast cancer.
They added: "The modest associations that have been observed for dietary fat and breast cancer risk in observational studies and clinical trials stand in sharp contrast to the robust evidence for a strong link between [body fat] and the risk of postmenopausal breast cancer."
Dr Emma Pennery, nurse consultant at the UK charity Breast Cancer Care, said: "The effects of a high fat diet on a woman's risk of developing breast cancer are still uncertain.
"Whilst this research adds to existing evidence in this area, other studies have not reached the same conclusions so we are still some way off understanding its exact influence."
She added: "However a high fat diet can lead to weight gain and it is widely accepted that being overweight, particularly after the menopause, does increase the risk of breast cancer."
Dr Sarah Rawlings, head of policy and information at Breakthrough Breast Cancer added: "Whether you have been through the menopause or not, being overweight is associated with a variety of health problems, including increased risk of heart disease, high blood pressure and several types of cancer.
"Maintaining a healthy weight throughout life can help reduce the risk of many diseases as well as promoting good health."