However, the protection did not kick in until the women had eaten less fat for four years.
Until now, the only known prescription against ovarian cancer -- aside from surgically removing the ovaries -- was to use birth control pills.
Use for five years can lower the cancer risk by up to 60%, protection that lingers after pill use ends.
The new findings now offer an option for postmenopausal women to try as well.
Those who followed a low-fat diet for eight years cut the chance of ovarian cancer by 40%, researchers reported Tuesday.
It is arguably the most promising finding of the Women's Health Initiative dietary study, which enrolled tens of thousands of healthy women 50 to 79 to track the role of fat in several leading killers.
Some women were assigned to cut the fat in their diets to 20% of calories -- from an average of 35% -- while others continued their usual diets.
Yet the study so far has found the diet made little impact on rates of breast cancer, colorectal cancer and heart disease.
From the October 22, 2007, Prepared Foods e-Flash