A US study, by University of Southern California researchers, said women at risk of endometriosis should be closely monitored for ovarian cancer.
What did the study find?
Researchers analysed data from 13 case studies of 23,144 women: 7,911 with invasive ovarian cancer, 1,907 with borderline ovarian cancer and 13,326 controls.
They examined the link between a history of endometriosis, found in 9.3% of women with invasive ovarian cancer, and cancer risk. Age, ethnic origin, oral contraceptive use and the number of times women had given birth were factored in.
Researchers found ovarian cancer risk varied by subtype. Women with endometriosis were two to three times more likely to develop invasive low-grade serous, endometrioid or clear-cell ovarian cancer.
A history of endometriosis was reported by 20.2% of women with clear-cell, 13.9% with endometrioid and 9.2% with low-grade serous subtypes of invasive ovarian cancer.
A history of endometriosis was not associated with a risk of mucinous or high-grade serous invasive ovarian cancer.
How significant are the results?
The authors wrote: 'Although we have reported strong associations between endometriosis and risk of low-grade serous, clear-cell and endometrioid ovarian cancers, most women with endometriosis do not develop ovarian cancer.'
They called for more research into how endometriosis can become malignant.
However, Dr Paul Pharoah, a Cancer Research UK-funded researcher at the University of Cambridge, said the findings had limited clinical relevance.
Although endometriosis increases relative risk of developing cancer, absolute risk remains low. Dr Pharoah said endometriosis increased lifetime risk of developing ovarian cancer from around one in 50 to one in 40.
But Dr Emily Power, Cancer Research UK health information manager, said: 'Ovarian cancer can be difficult to detect, so identifying women at higher risk could help doctors to develop more targeted monitoring.'
The researchers said ovarian surgery had been found to reduce risk of cancer linked to endometriosis. But Dr Pharoah added that surgical removal of the ovaries would only be offered to women at higher risk.