Women who have endometriosis appear to have a higher risk of developing several different kinds of cancer, say researchers.
One in four women has endometriosis
Endometriosis is a condition in which the type of tissue that lines the inside of the womb is found elsewhere in the pelvis.
Since the natural menstrual cycle of a woman involves the swift growth, then shedding of the womb lining during her period, this is not beneficial.
Typical symptoms include pelvic pain, heavy menstrual bleeding, bloating and fatigue.
Where there are slightly increased risks, they are in some of the less common cancers
Dr Anna-Sofia Berglund, Huddinge University Hospital
It has also been linked with difficulty conceiving.
Researchers from Huddinge University Hospital in Stockholm, Sweden, looked at whether there was a link between having endometriosis and cancer risk.
They found a woman's risk of developing ovarian cancer increased by just under half, for endocrine tumours by a third, for non-Hodgkin's lymphoma approximately a quarter and for brain tumours just over a fifth.
However, the risk of cervical cancer fell by roughly a third.
The author of the study, presented at the annual meeting of the European Society for Human Reproduction and Embryology in Madrid, said that as these were relatively uncommon cancers, even apparently large increases in lifetime risk were not necessarily anything to be concerned about.
Dr Anna-Sofia Berglund said: "It is very important to keep these findings in perspective.
"The overall risk of cancer does not increase after endometriosis, and where there are slightly increased risks, they are in some of the less common cancers.
"For instance, in Sweden just under 20 women in every 100,000 develop ovarian cancer each year.
"My study shows that for women with endometriosis, another eight women in 100,000 could develop it - and it may be even fewer than that."
The study found that women who had a hysterectomy before or at the time that endometriosis was diagnosed did not show this increased risk of ovarian cancer - suggesting a preventive effect.
Dr Berglund said the study did not prove endometriosis caused cancer - but that it was possible that whatever led to endometriosis might increase the risk.