Norweigan doctors now say they've found that some tumors may spontaneously disappear without treatment.
They studied 200-thousand women - one-half got mammograms twice a year, while the others got them less often.
They found that overall breast cancer rates were 20-percent higher among those who got mammograms the most frequently.
Their conclusion is the mammograms were picking up some invasive cancers that would ultimately "go away."
But since there's no way to tell which tumors would vanish, so doctors say it's safest to treat them all.