Scientists were able to see a direct link between reduced hormone therapy and decline in rates of breast cancer.
The researchers believe they also have uncovered indirect evidence that hormones promote breast tumor growth.
The declines occurred in the age groups that most widely embraced then abandoned hormone therapy.
Numerous studies have suggested that women taking a combination of progestin and estrogen faced a higher risk of breast cancer and other potential health hazards.
The new findings suggested that hormones helped promote breast tumour growth of pre-existing, clinically latent hormone-dependent cancers, not only increasing the incidents of invasive cancer, but also the risk of ductal carcinoma in situ (DCIS).
"We show that the incidence of breast cancer decreases if you take the hormones away,'' said senior author Karla Kerlikowske.
"The fact that we're continuing to see a decrease in invasive cancer means that the effects of stopping the hormones may be long-lasting," she added.
The study included nearly 700,000 women, aged 40 to 79, who underwent more than 2 million screening mammography examinations between January 1997 and December 2008.
The researchers found that women aged 50 to 69 had the highest level of hormone therapy use and also had the largest reduction in invasive breast cancer when they stopped hormone therapy - from 40 cancers per 10,000 mammograms in 2002 to 31 cancers in 2005, and 35 cancers in 2006.
The study also found a sharp drop in rates of ductal carcinoma in situ in this age group after they stopped using hormone therapy.
A parallel decrease in these types of breast cancer also occurred among women older than 70. However, there was no change in breast cancer rates during the study period among women aged 40 to 49, who were less likely to have been on hormone therapy.
"The study supports the idea that by giving the hormones we were promoting tumor growths by giving the hormones. When the promoter is taken away, the incidence of breast cancer decreases," said Kerlikowske.
The Journal of Clinical Oncology has published the study online.