Int. J. Environ. Res. Public Health 2011, 8 413 associated with increasing SES [8,26,28]. Socioeconomic disparities have also been observed in receptivity toward such policies, with individuals of lower SES being less likely to support restrictions in indoor public places and worksites [29]. There is also evidence to suggest that the benefits of smoke-free policies may be irrespective of socioeconomic status; one recent study found that comprehensive policies prohibiting smoking in all worksites, restaurants, and bars are independently associated with reduced smoking participation and consumption, regardless of education and income [30]. Given the magnitude and persistence of the association between SES and tobacco use, the reduction of social inequalities in smoking-related indicators has become an important public health priority for many national governments and policymakers [31]. A key strategy used to achieve this goal has been the implementation and intensification of smoking restrictions in indoor environments [3]. However, there has been limited research on the extent to which these restrictions vary among socioeconomic groups both across and within countries. More specifically, it is of particular importance to determine whether the recent worldwide proliferation of smoke-free policies [4,5] has been uniform across socioeconomic groups. The primary objective of this study was to utilize population data from Canada, the United States (US), the United Kingdom (UK), and Australia to assess socioeconomic and national variations in the prevalence, introduction, retention, and removal of smoke-free policies in various indoor environments, including homes, worksites, bars, and restaurants. The assessment of policy change was of particular interest in the UK, where national legislation prohibiting smoking in worksites, bars, and restaurants was implemented among most respondents between data collection Waves. A secondary objective was to identify sociodemographic predictors of these policy-related indicators by environment type. Understanding sociodemographic disparities in access to smoke-free environments may help mitigate such disparities in tobacco use. 2. Methods This study reports data from Waves 5 and 6 of the International Tobacco Control Four Country Survey (ITC-4), a prospective cohort study to monitor the impact of national level tobacco control policies in four countries: Canada, the US, the UK, and Australia. The ITC-4 was initiated in 2002 with over 2,000 adult smokers recruited by probability sampling methods in each of the four countries. In subsequent follow-up surveys of the cohort, recruited samples are replenished after attrition to ensure a sample size of at least 2,000 per country at each Wave. A detailed description of the design features, data collection methods, and analytic strategies of the ITC-4 study have been previously reported elsewhere [32]. Participants were identified using stratified random digit dialing and interviews were conducted using computer assisted telephone interview (CATI) software by multiple research firms. The interviews were conducted in either English or French (francophone areas of Canada), but strict protocols were followed to ensure methodological congruity across firms and between the two languages [32]. The study protocol received ethical review and clearance from the Institutional Review Boards or Research Ethics Boards at the University of Waterloo (Canada), Roswell Park Cancer