The research suggests that these screening X-rays do not lower the risk of dying of breast cancer while finding many tumors that do not need treatment.
The study gives longer follow-up on nearly 90,000 women who had annual breast exams by a nurse to check for lumps plus a mammogram, or the nurse's breast exam alone. After more than two decades, breast cancer death rates were similar in the two groups, suggesting little benefit from mammograms.
It's important to note that this study did not compare mammograms to no screening at all, as most other research on this topic has.
Many groups have not endorsed breast exams for screening because of limited evidence that they save lives.
The American Cancer Society says combined data from past trials shows that mammograms reduce the death rate by at least 15 per cent for women in their 40s and by at least 20 per cent for older women.
About 37 million mammograms are performed every year in the United States, including three-quarters of the women 40 and older.
Critics of the Canadian study also say it used outdated equipment and poor methods that made mammograms look unfairly ineffective.
The study was published Wednesday in the British journal BMJ.