A drop in breast cancer cases may be due to women eschewing Hormone Replacement Therapy (HRT), experts say.
HRT has been linked to a risk of cancer
University of Texas researchers recorded a 7% drop in new breast cancer cases in the US in 2003.
The told a US cancer conference the fall could be linked to the fact that millions of women gave up HRT following reports questioning its safety.
They recorded an even bigger fall - 12% - in cases of hormone-dependent breast cancer among women aged 50-69.
UK researchers say they have also seen a drop in breast cancer cases in women in their early 50s.
Women in their 50s and 60s are those most likely to use HRT.
The researchers told the San Antonio Breast Cancer Symposium around 14,000 fewer US women were diagnosed with breast cancer in 2003, compared with the previous year.
In 2002 a large study of women using HRT was halted after evidence emerged that the form of therapy being used apparently increased the risk of developing breast cancer.
The researchers said the number of American women on HRT had halved by the end of 2002 in the wake of the scare.
Researcher Dr Peter Ravdin said: "It is the largest single drop in breast cancer incidence within a single year I am aware of.
"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 team suggested that the figures could be explained by existing tumours stopping growing, shrinking or disappearing so that they could not be detected.
Dr Donald Berry, who also worked on the study, said: "It takes breast cancer a long time to develop, but here we are primarily talking about existing cancers that are fuelled by hormones and that slow or stop their growing when a source of fuel is cut.
"These existing cancers are then more likely to make it under mammography's radar."
Professor Valerie Beral, director of Cancer Research UK's Cancer Epidemiology Unit, said there had also been a slight drop in breast cancer incidence in UK women aged 50-64 between 2003 and 2004.
She stressed women who had been using HRT should not worry.
"Although HRT increases the risk of breast cancer while it's being used, once women stop taking it the risk falls away quite quickly."
Dr Sarah Cant, of the charity Breakthrough Breast Cancer, said there was not enough information to say that a decline in HRT use had led to the decrease.
She said: "Breast cancer is a complex disease and little is known about its causes.
"Although we've known for some time that taking HRT can increase the risk of developing breast cancer, short-term use is likely to have only a small effect.
"We advise women to weigh up the risks and benefits of taking HRT with the help of their GP."
Pamela Goldberg, chief executive, Breast Cancer Campaign said further data was needed from other countries before firm conclusions could be made.