New research from Denmark suggests many women may be having unnecessary treatment for breast cancer.
The ABC reports the study shows as many as one third of cancers that show up in screening tests may pose no threat to health.
But because it is not possible to tell lethal cancers from harmless ones, all those detected are treated the same way.
Dr Karsten Jorgensen of the Nordic Cochrane Centre, who led the research, says some women are having harmful treatment they do not actually need.
Researchers took information from screening programmes in Australia, Britain, Canada, Norway and Sweden between 1971 - 1999. Their results have been published in the British Medical Journal.