Researchers followed 67,470 women aged 34 to 59 at the start of the study from 1980 to 2006. The women recorded dietary details, lifestyle information and medical history. The study was published online Nov. 22 in Cancer Epidemiology, Biomarkers & Prevention.

After controlling for dozens of other factors, the scientists found that compared with women who drank less than one cup of coffee a day, those who drank four or more cups had a 25 percent lower risk of endometrial cancer. Neither decaffeinated coffee nor tea drinking was associated with a risk reduction, and the authors drew no conclusions about whether caffeine or some other ingredient in coffee causes the effect.

The lead author, Youjin Je, a doctoral student at the Harvard School of Public Health, said that for healthy non-pregnant women, drinking four cups of coffee a day has no known negative effects.

But, she said, “a substantial amount of sugar, cream or milk added to coffee can negate the potential benefits.”