New research claims women who take the contraceptive pill receive "substantial long term protection" against ovarian cancer.
The study published in The Lancet also found that women have less chance of developing the disease the longer they use the Pill.
And even when they stop using it, the protection lasts, as women who took the contraceptive for 15 years halved their risk of ovarian cancer.
Professor Valerie Beral, director of the Cancer Research UK Epidemiology Unit at Oxford University, said: "Worldwide, the Pill has already prevented 200,000 women from developing cancer of the ovary, and has prevented 100,000 deaths from the disease.
"More than 100 million women are now taking the Pill, so the number of ovarian cancers prevented will rise over the next few decades to about 30,000 per year."
Co-author of the report Sir Richard Peto, Professor of Epidemiology at Oxford, added: "Young women don't have to worry about cancer from taking the Pill, because the eventual reduction in ovarian cancer is bigger than any increase in other types of cancer caused by the Pill."
And Dr Lesley Walker from Cancer Research UK said: "All women who have taken the Pill or are currently taking it should be reassured by this study."