The study involving 80,000 women suggests screening saves the lives of two women for every one that may have a false alarm.
Mammograms can spot dangerous tumors. But, they can also detect lumps that may be harmless exposing women to anxiety and unnecessary surgery.
There is now competing research questioning whether routine screening is worth the trouble and expense.
Just last week a study in the British Medical Journal found no evidence that routine screening for breast cancer had any effect on death rates.