Correspondence

doi: 10.1002/cam4.304 Several recent cohort studies have found an association between smoking and breast cancer, but the association between lifetime smoking exposure and breast cancer mortality is less well described. We examined whether smoking before breast cancer diagnosis is a predictor of breast cancer mortality in a large cohort with more than 4.1 million years of follow-up, with a special focus on women who initiated smoking before first childbirth. Information on smoking status was collected before breast cancer diagnosis and used to estimate hazard ratios (HRs) and corresponding 95 % confidence intervals (CIs) of breast cancer mortality in a cohort of 302,865 Norwegian women with 1106 breast cancer deaths. Women were enrolled between 1974 and 2003 and followed up through linkages to national registries until 31 December 2007. We found that breast cancer mortality was slightly but significantly increased for current (HR = 1.15, 95 % CI 1.01–1.32) and ever (HR = 1.15, 95 % CI 1.02–1.30) smokers as com