Many ErbB2-positive cancers may show intrinsic resistance, and the frequent development of acquired resistance to ErbB-targeted agents represents a substantial clinical problem. The constitutive NF-κB activation in some HER-2/neu positive breast cancer may represent a potential cause of resistance to trastuzumab therapy. Preclinical data revealed that 4-(N-Methyl-N-nitrosamino)-1-(3-pyridyl)-1-butanone (NNK), the tobacco-specific nitrosamine is able to enhance NF-κB DNA binding activity and theoretically to increase the resistance to trastuzumab. Two hundred and forty-eight women with pathologically confirmed, uni- or bidimensionally measurable, HER-2-positive metastatic breast cancer (MBC) treated with trastuzumab-based therapy as first line combination for metastatic disease were considered eligible. For all included patients data on smoking habit were detectable from medical records. We retrospectively analysed the smoking habits of 248 MBC patients and correlated these habits with activity and efficacy of trastuzumab-based therapy. No statistically significant difference in terms of response rate (RR), time to progression (TTP) and overall survival (OS) was identified between smokers (former plus active smokers) and never smokers. Moreover, no statistically significant difference in terms of RR, TTP and OS was identified either comparing active smokers and former smokers. Moreover, we did not observed any significant statistical difference in terms of TTP and OS between smokers ≥10 cigarettes/day and <10 cigarettes/day. This study clearly showed lack of any correlation between cigarette smoking habit and both activity and efficacy of trastuzumab-based first line therapy in metastatic HER2/neu positive breast cancer patients.