Background: Exposure to second-hand smoke (SHS) after a cancer diagnosis is associated with continued smoking in lung and head and neck (HN) cancer patients (PMID: 24419133, 23765604). We evaluated whether complete reduction/cessation 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 design). These cancers were chosen because these patients had the highest rates of smoking at the time of diagnosis. Multivariate logistic regression analysis evaluated the association of cessation of SHS exposure after a diagnosis of cancer with subsequent smoking cessation, adjusted for significant covariates. A cross-sectional study (2014) of 90 lung and HNC smoking patients assessed consistency in associations. Results: For the cohort, 261/731 lung and 145/450 HN cancer patients smoked at diagnosis; subsequent overall 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. Cessation 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 in both cancers combined (aOR = 5.00 [3.03-8.33], P< 0.001). The cross-sectional study had a similar trend for cessation of SHS with smoking cessation, but a lower magnitude of association (OR = 2.73, P= 0.09). However, when asked directly, only 13% of patients quit smoking with another individual. Conclusions: Cessation of SHS exposure was a frequent occurrence around cancer patients. This cessation of SHS exposure is significantly associated with smoking cessation in lung and HN cancer patients. However, few patients quit smoking at the same time as their friends, family or household. Changing the environment around cancer survivors to reduce SHS exposure and encouraging households/friends to quit smoking may both improve cessation rates in cancer patients.