BMC Cancer 2008, 8:157 http://www.biomedcentral.com/1471-2407/8/157 As mentioned earlier, there was also non-participation among cases. If smoking was related to a more aggressive form of breast cancer, a greater proportion of cases who smoked may have been excluded by physicians for medi- cal reasons, resulting in a bias toward the null. However, the overall percentage of cases diagnosed at advanced stages was small, and stage distribution was similar across participants and non-participants. Alternatively, the associations with breast cancer risk observed for quantita- tive measures of active smoking and secondhand smoke exposure may be due to negative confounding by unmeas- ured factors, although it is difficult to speculate what these confounders could be. Non-differential misclassification of tobacco smoke exposures could have also biased the results toward the if both the cases and the controls had equal difficulty recalling their lifetime exposures to tobacco smoke. Conclusion In conclusion, exposure to tobacco smoke, either through active or secondhand smoking, was not associated with breast cancer risk in this Delaware population. Women should still be encouraged to avoid tobacco smoke expo- sure to prevent other adverse health effects associated with both active and secondhand smoking. Competing interests The authors declare that they have no competing interests. Authors' contributions DER participated in the coordination of the study, per- formed the statistical analyses and drafted the manuscript. RCB and HLH participated in the design of the study and helped to draft the manuscript. CJN conceived of the study, participated in its design and coordination, and helped to draft the manuscript. All authors read and approved the final manuscript. Additional material Additional file 1 Secondhand smoke exposure at home among non-smoking breast cancer cases and controls from Delaware. Table. Click here for file [http://www.biomedcentral.com/content/supplementary/1471- 2407-8-157-S1.doc] Acknowledgements This study was supported in part by the American Cancer Society. DER was supported by grant 2 T32 CA093-14-19 from the National Cancer Institute. We thank the Delaware Cancer Registry for providing information on breast cancer cases, especially the perseverance of Colleen Ferber. We also thank Dr. Edward Ratledge, Roberta Gibson, Jo-Ell Malloy and all of the dedicated interviewers at the University of Delaware Center for Applied Demography and Survey Research for their contributions to control selec- tion and data collection. The authors acknowledge the cooperation of Del- aware physicians who allowed us to contact their patients. References 1. Cancer Facts and Figures 2008. Atlanta, GA, American Cancer Society; 2008. 2. 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