ABSTRACT: OBJECTIVES: To examine the association between time of smoking initiation and both the independent and joint effects of active and passive tobacco smoke exposure and the risk of breast cancer in a sample of Ontario women. METHODS: Data from two large population-based case-control studies conducted among Ontario women aged 25-75 years were combined for analysis (n = 12,768). RESULTS: Women who had ever smoked and were exposed to passive smoke had a significant increased risk of breast cancer (OR 1.13, 95%CI 1.01-1.25). A significant increased risk was also observed among women who initiated smoking: at age 26 or older (OR 1.26, 95%CI 1.03-1.55); more than five years from menarche (OR 1.26, 95%CI 1.12-1.42); and, after their first live birth (OR 1.25, 95%CI 1.02-1.52). CONCLUSION: The results suggest that women who initiate smoking at an older age are at an increased risk of breast cancer.