This finding was published online Monday on the "Journal of Clinical Oncology" in the United States.

　　The study was done by the City of Hope, a Comprehensive Cancer Center in Duarte, California.

　　The researchers collected data from more than 4,500 U.S. women in the 35-64 age group who were diagnosed with breast cancer.

　　With the passing of  more than eight years, the researchers found the black women have a three times death rate than white women, after taking obesity and other diseases into account.

　　"It’s been long known that breast cancer in African-American women is a far less common disease than in white women. But when it occurs, it seems to be more aggressive and harder to treat," said Dr. Lisa Carey of the University of North Carolina’s Lineberger Comprehensive Cancer Center.

　　But why the situations varied by race, scientists are still exploring the answer.