The research found after five years, breast cancer had returned in 12.4 % of those on a standard diet - but in only 9.8% of those on a low-fat diet.
However, most women did not benefit, and experts say the findings may be due to other factors.
The Los Angeles Biomedical Research Institute study was presented to the American Society of Clinical Oncology.
The study focused on 2,437 women who had surgery for early-stage breast cancer, followed by standard chemotherapy, and tamoxifen if their tumours were receptive to the drug.
On average, the women already followed a healthy diet, gaining 29% of their calories from fat.
Doctors told 1,462 of them to continue their normal diets, averaging 51.3 grams of fat a day.
The rest were given help to follow a low-fat diet, averaging just 33.3 grams of fat a day.
Overall, the low-fat group had a 24% lower risk of the disease coming back.
Hormone positive cancer
However, the only women who benefited were those whose tumours were not helped to grow by the female sex hormone oestrogen.
These women had 42% lower risk of recurrence if they ate low-fat diets - but they accounted for just one in five women in the study, similar to breast cancer cases in the general population.
Results for the other 80% of women in the study did not reach statistical significance - meaning they could have occurred by chance alone.
Dr Len Lichtenfeld, of the American Cancer Society, said the results did not mean that all women with breast cancer should be advised to follow a low-fat diet.
Others noted that women in the low-fat group lost on average four pounds (1.8kg), and that many studies have linked excess weight to excess breast cancer risk.
The low-fat dieters also likely ate more fruits and vegetables and less red meat - other things known to lower breast cancer risk.
Even lead researcher Dr Rowan Chlebowski admitted: "We can't separate those components out."
Dr Eric Winer, of the Dana-Farber Cancer Institute in Boston, also urged caution.
He said: "There are more questions than answers.
"What we don't want to happen is for every woman who's had breast cancer to panic if she's had a Big Mac."
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