Objective To examine the influence of hormone-related factors on the risk of invasive epithelial ovarian cancer (ovarian cancer). Design Population-based ca se-control study using in-person interviews. Setting Academic department of pr eventivemedicine. Patient(s) Four hundred seventy-seven ovarian cancer patients and 660 controls. Intervention(s) None. Main outcome measure( s) Numbers of and ages at births, oral contraceptive use, and use of menopausal hormone therapy. Result(s) Compared with nulliparous women, women whose only (last) birth was aft er age 35 years had an estimated 51%(95%confidence interval: 21%-70%) reduc tion in risk. If this birth occurred earlier, the reduction in risk was progress ively less. Additional (earlier) births reduced the risk further. Oral contracep tive use also reduced risk. Increased body mass index increased risk, but this e ffect was confined to localized disease and is likely to be a diagnostic bias, a s a consequence of other problems associated with being overweight and in itself having no etiological significance. Conclusion(s) If the major protective effec t of a late birth can be confirmed, our most challenging task will be to underst and the mechanism to develop a chemoprevention approach to exploit this finding. Objective To examine the influence of hormone-related factors on the risk of invasive epithelial ovarian cancer (ovarian cancer). Design Population-based ca se-control study using in-person interviews. hundred seventy-seven ovarian cancer patients and 660 controls. Intervention (s) None. Main outcome measure (s) Numbers of and ages at births, oral contraceptive use, and use of menopausal hormone therapy. women whose only (last) birth was a pediatric age 35 years had an estimated 51% (95% confidence interval: 21% -70%) reduc tion in risk. If this occurs occurred earlier, the reduction in risk was progress ively less. Additional (earlier) births reduced the risk further. Oral contracep tive use also reduced risk. Increased body mass index increased risk, but this e ffect was confined to localized disease and is likely to be a diagnostic bias, asa consequence of other problems associated with being overweight and in itself having no etiological significance. Conclusion (s) If the major protective effec t of a late birth be able, our most challenging task will be to underst and the mechanism to develop a chemoprevention approach to exploit this finding.