Although the use of oral contraceptives (OCs) is not generally associated with increased risk of breast cancer, higher risks have been reported for some subgroups of users. We have carried out a population-based case-control study in the Netherlands to assess the effect of timing and duration of OC use on the risk of breast cancer developing at various ages. 918 women with breast cancer (20-54 years at diagnosis) were pair-matched by age with controls randomly selected from municipal registries. Information on OC use obtained from women and their prescribers was combined according to standard decision rules. Overall, long-term use of OCs (> or = 12 years) had an associated relative risk (RR) of 1.3 (95% CI 0.9-1.9; test for trend in risk with months of use p = 0.03). This positive trend was found in both the youngest (< 36 years; p = 0.08) and the oldest (46-54 years, p = 0.004) age groups, but not in women aged 36-45 years. The RR of developing breast cancer before age 36 was 2.1 (1.0-4.5) for 4 or more years of OC use compared with shorter use. In women younger than 36, risk increased for longer OC use before age 20 (1.44 per year, p = 0.04). Recent use (previous 3 years) was associated with increased risk in women of 46-54 (RR 1.9 [0.9-4.1], p = 0.02). We conclude that 4 or more years of OC use, especially if partly before age 20, is associated with an increased risk of breast cancer developing at an early age. There is limited evidence that the excess risk disappears as the cohort of young OC users ages, but this issue needs confirmation.