A Canadian study has found that mammograms don't give women in their 40s a better chance of surviving breast cancer.

Researchers at the University of Toronto monitored a group of 50,000 women, aged 40 and 49, over a 13-year period. None of the women, who lived in 15 centres across Canada, had been diagnosed with breast cancer at the start of the study.

The study found mammography had little effect on reducing the number of deaths.

Mammograms are X-rays of the breast tissue, used to screen for breast cancer in women before symptoms occur. Early detection is believed to increase a patient's chance of beating the disease.

But the Canadian study, published in the journal Annals of Internal Medicine, found the test didn't have a significant effect on mortality rates of women younger than 50.

An estimated 20,500 Canadian women are diagnosed with breast cancer each year, and about 5,400 women die every year in Canada from the disease, according to the Canadian Breast Cancer Federation.

Subjects were divided into two groups -- half received annual mammograms and clinical breast exams, while the other half received only breast exams once a year. But though doctors caught more cancer cases, especially smaller tumours, in the group that received mammograms, the number of breast cancer deaths were the same for both groups.

The study is the latest in a battery of confusing and inconclusive looks at breast health.

In the same issue of Annals, U.S. researchers concluded that mammograms could, in fact, reduce death rates from breast cancer slightly, particularly in women over 50. Another study, released by the World Health Organization earlier this year, concluded that regular mammograms cut breast cancer deaths in women aged 50 to 69 by 35 per cent. Other studies have found that mammograms can yield a reduction in breast cancer's mortality rate by as much as 55 per cent.

Researchers have found similarly conflicting evidence about the efficacy of breast self examinations.

Still, Dr. Cornelia Baines, one of the authors of the Canadian study, is convinced that mammograms aren't the answer for women in their 40s.

"Women are very frightened by breast cancer and they are very keen to have a solution to the breast cancer problem," she told CTV News. But she says her findings have convinced her that "mammography is not the solution to their problem."

Canadian guidelines currently recommend mammograms for women over 50, but cancer care experts say the latest findings don't rule out mammograms for some younger women.

"When it comes to making a decision about being screened or not, again, I think women need to decide together with their doctors," says Dr. Verna Mai of Cancer Care Ontario.