New federal statistics provide powerful evidence that the sharp drop in hormone use by menopausal women that began in 2002 caused a dramatic decline in breast cancer cases, according to an analysis being published today.

The statistics show that the number of breast cancer cases being diagnosed began falling abruptly after concerns emerged about the safety of hormone treatment and that the decrease persisted into the following year, strengthening the case that the trends are related, researchers said.

"At first I didn't believe it -- it was so astounding," said Donald A. Berry of the University of Texas, who led the analysis published in the New England Journal of Medicine. "But it really looks like it's a story that holds together."

The researchers estimated from the findings that about 16,000 fewer cases of breast cancer are being diagnosed each year because of the decrease in hormone use, a stunning reversal of a decades-long increase in cases.

"This is colossal," said Rowan Chlebowski of Harbor-UCLA Medical Center, who helped conduct the analysis. "It translates into thousands of fewer breast cancers that have been diagnosed in women in the United States and could be in the future."

The findings also help explain one of the biggest mysteries about breast cancer -- why the number of cases rose steadily for decades. Increasing hormone use probably played a key role, along with better detection by mammography and other factors, several experts said.

"I think this solves at least part of the mystery," Berry said.

Others said the findings underscore the danger of drug therapies becoming widely used before they have been thoroughly tested.

"An awful lot of breast cancer was caused by doctors' prescriptions," said Larry Norton of Memorial Sloan-Kettering Cancer Center in New York. "That's a very serious and sobering thought."

Norton and others said the findings should encourage more women to stop hormone use altogether or to continue at the lowest dose and for the shortest time necessary.

The findings come as another study involving nearly 1 million British women found that hormones also increase the risk of ovarian cancer.

"These data add to the message that we really should be discouraging women from initiating menopausal hormones," said Marcia L. Stefanick of Stanford University. "We need to stop underplaying those risks. They are very real."