A new study has found annual mammography screening of women in their 40s reduces the breast cancer death rate in those women by nearly 30 per cent.

The landmark breast cancer screening study of women aged 40 to 49 has been published online in the journal Cancer.

Around the world, most women begin mammogram screening at the age of 50. In Australia women are offered free mammograms from 50 to 69 years of age.

But the authors of the new study say the use of the age of 50 as a threshold for breast cancer screening is scientifically unfounded.

"Women should begin getting annual mammograms at age 40," the authors said.

"This study, which looked at the performance of screening mammography as it is actually used, rather than relying on mathematical modelling, shows without a doubt that mammography decreases deaths from breast cancer in women aged 40-49 by nearly one third," said Dr Carol H Lee, chair of the American College of Radiology Breast Imaging Commission.

"There is no excuse not to recommend that average-risk women begin annual screening mammography at age 40."

Younger women are not routinely offered mammograms because their breast tissue is more dense and lumps are harder to detect. They are more likely to be offered breast ultrasounds.

"This study shows that annual mammograms for women 40 and over result in a tremendously significant reduction in the breast cancer death rate for women 40 to 49," said Dr Phil Evans, president of the Society of Breast Imaging.

"The age of 50 is an artificial threshold that has no basis in scientific fact. The debate is now over.

"Women should no longer be confused about the importance of annual breast cancer screening. Mammography saves lives. If you are a woman age 40 or over, one of them could be yours."

The trial followed more than 600,000 women for 16 years.

There were 803 deaths of women from breast cancer in parts of Sweden where they offered screening to women from the age of 40, while there was 1238 deaths in areas where women received screening from the age of 50.

In Australia, women over 50 are offered mammograms every two years.

Breast cancer is the second most common cause of cancer-related death in Australian women - 2,618 Australian women died from breast cancer in 2006.

The lifetime risk of women developing breast cancer before the age of 75 years is one in 11.

Well-organised mammographic screening can substantially reduce deaths from breast cancer.

"It is now time to stop confusing women with conflicting information. Mammography is a lifesaver for women in their 40s," said Dr Gail Lebovic, breast surgeon and president of the American Society of Breast Disease.

"What providers need to do now is uniformly confirm for women that they need to start getting annual mammograms beginning at age 40 and work to build on the ability of mammography to detect cancer early, when it is most treatable.

"Mammography saves a significant number of lives in all women 40 and over. Let's all move forward from there."