Background: Exposure to SHS after a cancer diagnosis is associated with continued smoking in lung and HN cancer patients (PMID: 24419133, 23765604). However, smoking is a social activity. We evaluated whether elimination of SHS exposure around and after a diagnosis of lung or HN cancer is associated with smoking cessation in the cancer patient. Methods: Lung and HN cancer patients from Princess Margaret Cancer Centre (2006-12) completed questionnaires at diagnosis and follow-up (median 2 years apart) that assessed smoking history and SHS exposures (cohort study). Multivariate logistic regression analysis evaluated the association of elimination of SHS exposure after a diagnosis of cancer with subsequent smoking cessation, adjusted for significant covariates. A cross-sectional study (2014-15) of 183 lung and HN smoking patients assessed consistency in associations and interest in SHS cessation programs. Results: For the cohort study, 261/731 lung and 145/450 HN cancer patients smoked at diagnosis; subsequent quit rates were 69% and 50% respectively. 91% of lung and 94% of HN cancer patients were exposed to SHS at diagnosis while only 40% (lung) and 62% (HN) were exposed at follow-up. Elimination of SHS exposure was associated with smoking cessation in lung (aOR = 4.76, 95% CI [2.56-9.09], P< 0.001), HN (aOR = 5.00 [1.61-14.29], P< 0.001), and combined cancers (aOR = 5.00 [3.03-8.33], P< 0.001). The cross-sectional study has similar cessation and SHS exposure rates and a similar association for elimination of SHS with smoking cessation (aOR = 3.42 [1.16-10.10], P= 0.03). However when asked directly, only 26% of patients quit smoking with another individual and 13% of patients exposed to SHS had at least 1 interested party in joining a SHS cessation program. Conclusions: Elimination of SHS exposure around patients is significantly associated with smoking cessation in lung and HN cancer patients, but few patients quit smoking together with others around them, despite the ‘teachable moment’ with a cancer diagnosis. Clinicians should encourage patients and their household/friends to quit smoking together to improve cessation rates in cancer patients and those around them.