Benign proliferative epithelial disorders (BPED) of the breast are conditions which are thought to have premalignant potential. The purpose of the present investigation was to study the association between cigarette smoking and risk of BPED. The study was undertaken within the 56,837 women within 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.) During the course of the follow-up period, a total of 691 women in the dietary cohort were diagnosed with biopsy-confirmed incident BPED. For comparative purposes, a subcohort consisting of a random sample of 5681 women (including 65 of the subjects with BPED) was selected from the full cohort. After exclusions for various reasons, the analyses were based on 691 cases and 5443 non-cases. Overall, the results of the present study are almost uniformly null, and provide little support for an association between cigarette smoking and risk of BPED. Analyses conducted separately in the screened and control arms of the NBSS also failed to provide strong support for associations, and there was little difference between the results for screen-detected and interval-detected BPED. There was some suggestion of an increase in risk in post-menopausal ex-smokers, but the trend in risk with cigarette-years of exposure in this group was not statistically significant. Risk in ex-smokers varied little by years since smoking cessation. There was little evidence for associations with either atypical or non-atypical forms of BPED.