Women who take hormone replacement therapy (HRT) are at an increased risk of developing lung cancer, new research suggests.

A large U.S study of 36,588 peri- and postmenopausal women found that those participants who took estrogen and progestin HRT were 50 per cent more likely to develop lung cancer than those women who did not take HRT.

Use of unopposed estrogen — that is, estrogen taken without an accompanying progestin — was not associated with an increased risk.

Researchers found that the longer women used HRT, the greater their chance of developing lung cancer and the more advanced the stage of the disease at the time of diagnosis.

In the study, a woman's smoking status, her age and other factors were taken into account in determining the risk of developing lung cancer from taking HRT.

The data used was collected between 2000 and 2002 through a study conducted in Washington State. Researchers from Oregon, Virginia, North Carolina and Washington State then tracked the women who met the study criteria for six years.

"Although HRT use has declined and is not recommended except for short-term treatment of menopausal symptoms, our results indicate millions of women may remain at risk of developing lung cancer," said Chris Slatore, principal investigator and an assistant professor of medicine in the Oregon Health & Science University School of Medicine, in a release.

The scientists aren't sure why HRT seems to raise a woman's lung cancer risk, speculating that estrogen coupled with progestin may lead to more aggressive forms of the disease or hide early symptoms.

The research was published Wednesday in the Journal of Clinical Oncology.