The findings were presented this week at a conference of the American Association for Cancer Research in Miami, the Los Angeles Times reported.
The research team questioned 97 black women who had recently been diagnosed with breast cancer and 102 women without cancer. They said women who had done at least 2 hours per week of vigorous exercise in the past year appeared to have a risk of breast cancer 64 percent less than those who had not, while women who engaged in moderate exercise had a 17 percent less risk.
Running, aerobics and similar activities were classified as vigorous exercise, while walking was considered moderate.
Women who had been through menopause appeared to get the greatest benefit from exercise.
"People often want to know what they can do to reduce their risk of disease," said Vanessa Sheppard, an oncologist at the Georgetown Lombardi Comprehensive Cancer Center in Washington and the lead author. "We have found that just 2 or more hours of vigorous activity per week can made a difference in one's risk of developing breast cancer."