WASHINGTON - Breast cancer may be different in many black American women than those of other races - more aggressive and of a harder-to-treat type, researchers said on Monday.

The study of more than 2,000 women seems to contradict theories that black women are more likely to die of breast cancer because they get poorer care or show up for treatment later.

Dr. Wendy Woodward of the University of Texas M. D. Anderson Cancer Center in Houston and colleagues compared the records of black, Hispanic and white breast cancer patients.

They found the black women had overall more aggressive tumors and were more likely to die than the Hispanic and white women.

Especially hard to treat are estrogen-receptor negative, or ER-negative, tumors, which are not fed by the hormone estrogen. These tumors are not affected by the most successful breast cancer drugs such as tamoxifen and the aromatase inhibitors, although some can be battled with the targeted cancer drug Herceptin.

"African-American women more frequently had ER-negative disease and high-grade tumors and ... African-American race was associated with a poorer survival rate," the researchers wrote in the journal Cancer.

Breast cancer is the second most common cause of cancer death in US women after lung cancer, and the No. 1 killer of women aged 45 to 55. Each year, 211,000 American women are diagnosed with breast cancer, and 40,000 die from it, according to the American Cancer Society, which publishes Cancer.