This is not the first time alcohol has been linked to breast cancer.
But previous studies showed no increased risk among light drinkers.
The new study found that even a single glass of wine a day can raise the danger.
"(In) the past we've known women who consume a lot of alcohol increase the risk of breast cancer," said Dr. Otis Brawley of the American Cancer Society. "We actually thought that a small amount of alcohol was relatively safe for breast cancer."
The study followed 100,000 nurses for nearly 30 years. It found that just three to six glasses of alcohol a week can increase the cancer risk by 15 percent.
And if you drink two glasses a day the risk mushrooms to 51 percent.
It made no difference whether women drank wine, beer or liquor.
The process works this way: alcohol increases estrogen which fuels tumor creation and growth.
But the picture is confusing because alcohol also has significant benefits for heart health.
"One in three women are going to die from heart disease and only one in 36 will die from breast cancer so it may be worth continuing to have that drink," said ABC News medical editor Dr. Richard Besser.
Doctors say that may apply as long as women don't have increased risk of breast cancer for other reasons.
The study appears in the Journal of the American Medical Association.
