Researchers estimated Wednesday that mammograms caused more than 1 million American women over the past three decades to be diagnosed with early-stage breast cancers that would not have proved fatal if left undetected and untreated -- a controversial finding sure to provoke more heated debate over the benefits of the screenings.

The study, published in the New England Journal of Medicine, concluded that these women were “overdiagnosed” and “overtreated” because there was not a corresponding decline in the diagnosis of tough-to-cure breast cancers that have already spread. The authors said mammography has saved few lives.