Endometrial and ovarian cancer and oral contraceptives--findings in a large cohort study.

Many case-control studies have shown that oral contraceptives protect against endometrial cancer and epithelial ovarian cancer, but little information is available from cohort studies. The findings from the Oxford Family Planning Association contraceptive study are reported here; the relative risks for ever users of oral contraceptives in comparison with never users were 0.1 (95% confidence interval 0.0-0.7) for endometrial cancer and 0.4 (95% confidence interval 0.2-0.8) for ovarian cancer. There was a strong negative relationship between duration of oral contraceptive use and ovarian cancer risk. Thus, in comparison with never users of oral contraceptives, the relative risk for users of up to 48 months' duration was 1.0 (95% confidence interval 0.4-2.5), while the relative risk for users of 97 months' duration or more was only 0.3 (95% confidence interval 0.1-0.7).