The risk of ovarian epithelial neoplasia following use of hormonal contraceptives (HC) was examined in data from the Norwegian-Swedish Women's Lifestyle and Health cohort including 103551 women aged 30-49 years in 1991-92. Follow-up through 2000 produced 214 incident cases of histologically confirmed epithelial ovarian neoplasias (135 invasive and 79 borderline cases). Using the Cox proportional hazard models, ever having used HC was associated with a decreased relative risk of epithelial ovarian cancer of 0.6 (95% CI 0.5-0.8). The effect of duration of HC use was convincing (P for trend <0.0001), and more important than age at start of use or time since first or last use. There was no significant difference between the effects of combined oral contraceptives and progestins-only contraceptives on risk (P=0.98). Similarly, there was no significant difference between the effects of ever use of HC on invasive and borderline ovarian neoplasia (P=0.37). In this cohort, use of HC seems to reduce the risk of epithelial ovarian neoplasia markedly and persistently in relation to the duration of use.