Mammograms did not detect any tumors among women under the age of 25, the researchers reported in the Journal of the National Cancer Institute.
The study of more than 117,000 U.S. women may reinforce controversial recommendations about the use of mammograms to screen for breast cancer among younger women.
Radiologist Bonnie Yankaskas of the University of North Carolina at Chapel Hill and colleagues examined the records of women aged 18 to 39 when they got their first mammograms starting in 1995, following them for a year to see what happened.
There were no tumors among the women under 25. For women aged 35 to 39, 12.7 per 1,000 got called back for further checks after the mammogram produced a suspicious-looking lesion. Very few actually had a tumor.
In a theoretical population of 10,000 women aged 35 to 39 years, 1,266 women who are screened will receive further workup, with 16 cancers detected and 1,250 women receiving a false-positive result, Yankaskas and colleagues wrote.
Harms need to be considered, including radiation exposure because such exposure is more harmful in young women, the anxiety associated with false-positive findings on the initial examination, and costs associated with additional imaging, they added.
About 29 percent of U.S. women aged 30 to 40 say they have had a mammogram.
Dr. Ned Calonge of the Colorado Department of Public Health and Environment said the findings suggest that women under 40 should not get mammograms unless they have symptoms of breast cancer, such as a lump.
Last November the U.S. Preventive Services Task Force, a federal scientific advisory panel, said women in their 40s with an average risk for breast cancer did not need annual mammograms to screen for the disease.
The guidelines touched off a debate among cancer doctors.
Calonge noted that most groups agree that women under 40 probably do not need mammograms to screen for cancer and Monday's study supports this approach.
Breast cancer is the most common cancer in women worldwide, accounting for around 16 percent of all female cancers. It kills around 465,000 people globally each year.