Incidence of the most common form of breast cancer dropped 15 percent from August 2002 to December 2003, U.S. researchers said Thursday.

The decline might be a result of decreased use of hormone replacement therapy, said researchers at the University of Texas M.D. Anderson Cancer Center in Dallas. Millions of older women stopped using HRT in 2002.

The researchers reported a 12 percent drop in cases diagnosed between 2002 and 2003 among women between 50 and 69 diagnosed with estrogen receptor positive (ER-positive) breast cancer -- the type of breast cancer most dependent on hormones for tumor growth.

They concluded that as many as 14,000 fewer women were diagnosed with breast cancer in 2003 than in 2002.

"It is the largest single drop in breast cancer incidence within a single year I am aware of," said Peter Ravdin, a research professor in the Department of Biostatistics at M.D. Anderson. "Something went right in 2003, and it seems that it was the decrease in the use of hormone therapy, but from the data we used, we can only indirectly infer that is the case."

The findings were presented at the Breast Cancer Symposium in San Antonio.