For every patient who undergoes "unnecessary treatment" after receiving breast cancer screening the lives of two women are saved, new research has revealed.
Some forms of the cancer will cause no harm to the sufferer and in other cases, cancers can grow so slowly that a woman may die from another condition first.
Currently all cases of cancer are treated, because medical experts are not able to differentiate between less harmful and those more aggressive variations.
Two lives are saved as a result of the NHS breast cancer screening programme for every case of overdiagnosis.
The issue has been the subject of debate recently after other studies suggested the programme does little to save lives. Last week, Danish experts cast doubt on the benefits of mammography, saying there are few differences in death rate between women who are screened and those who are not.
The latest research was led by experts from the Wolfson Institute for Preventive Medicine at Barts and the London School of Medicine and Dentistry.
It focused on data from some 80,000 women from the age of 50 and looked at Sweden and England before and after the introduction of screening.
It found 5.7 deaths from breast cancer were prevented for every 1,000 women screened over 20 years in England.
The number of estimated cases of overdiagnosis was 2.3 per 1,000 women over the same period.
The authors, writing in the Journal of Medical Screening, said: "The benefit of mammographic screening in terms of lives saved is greater in absolute terms than the harm in terms of overdiagnosis.
"Between 2 and 2.5 lives are saved for every overdiagnosed case."