The purpose of the cohort study reported here was to investigate the association between oral contraceptive use and risk of benign breast disease (BBD), overall and by histological subtype, within the 56,537 women in the Canadian National Breast Screening Study (NBSS) who completed self-administered lifestyle and dietary questionnaires. The NBSS is a randomized controlled trial of screening for breast cancer in women aged 40-59 at recruitment. Cases were the 2,116 women in the dietary cohort who were diagnosed with biopsy-confirmed incident BBD. For comparative purposes, a subcohort consisting of a random sample of 5,681 women (including 197 subjects with incident BBD) was selected from the full dietary cohort. After exclusions for various reasons, the analyses were based on 2,116 cases and 5,338 non-cases. There was an inverse association between use of oral contraceptives and risk of all types of BBD combined. The reduction in risk was confined largely to proliferative forms of BBD (BPED), and in particular, to those forms of BPED without histological atypia, in whom there was a progressive reduction in risk with increasing duration of use (the IRR (95% CI) for use of more than 7 years was 0.64 (0.47-0.87)); risk of BPED with atypia was increased somewhat in association with oral contraceptive use (the IRR (95% CI) for use of more than 7 years was 1.43 (0.68-3.01 )), but not in a dose-dependent manner. The results were similar when examined separately in the screened and control arms of the NBSS and for screen-detected and interval-detected BPED.