SAN ANTONIO — In a startling turnaround, breast cancer rates in America dropped dramatically in 2003, and specialists said they believe it is because many women stopped taking hormone pills.

The 7.2% decline came a year after a big federal study linked menopause hormones to a higher risk of breast cancer, heart disease, and other problems. Within months, millions of women stopped taking estrogen and progestin pills.

A new analysis of federal cancer statistics, presented yesterday at a breast cancer conference in Texas, showed the drop in tumors.

About 200,000 cases of breast cancer had been expected in 2003; the drop means that about 14,000 fewer women were diagnosed with the disease.

Because breast cancer takes years to form, specialists think that withdrawing hormones mostly caused small tumors that had been growing to stop or shrink, making them no longer detectable on mammograms. Whether this is true or will result in fewer cases over the long run will take more time to tell.

The next set of cancer statistics, for 2004, is due out in April.

Why do doctors think the 2003 drop is largely due to hormones?

Cases declined most among women 50 and older, with tumors whose growth is fueled by estrogen — the age group and type of cancer most affected by hormone use.

The drop also was seen in every single cancer registry that reports information to the federal government.

Researchers looked for a similar drop in other cancers, which could indicate something other than hormones was at work, “and we didn’t see anything,” said Kathy Cronin, a National Cancer Institute statistician who worked on the analysis.

When the 2003 numbers were first released a few months ago, they were grouped with 2001 and 2002 and portrayed as a leveling off of breast cancer after decades of steady rise. The big single-year drop was not pointed out because experts did not want to make too much of it without knowing whether the trend would continue.

However, Dr. Peter Ravdin, a breast cancer specialist at the University of Texas M.D. Anderson Cancer Center who led the new analysis, said the single year drop is important regardless, because it was so huge and came after years of steady increases.

“We don’t know about whether or not it’s going to be a trend but we know for this year it was a significant effect,” he said.

Doctors estimate that half of women who were taking hormones stopped after July 2002, when the federal Women’s Health Initiative study was halted because more women taking estrogen/progestin pills developed breast cancer or heart problems.