As a result of recent voter-approved initiatives, the sale of recreational marijuana commenced on January 1, 2014 in Colorado and should begin later this year in Washington.  Although many states have passed medical marijuana laws, the Colorado and Washington initiatives mark a dramatic change in US marijuana policy– from the complete outlaw of recreational sales, if not use, to the explicit sanctioning of it.  As of this writing, 13 other states in the United States are contemplating similar legalization initiatives. A key issue, however, is how these initiatives affect recreational and medical marijuana use. To ascertain these issues, we developed a panel survey that was first fielded in October 2013 in Colorado and Washington as well as two nearby states: Arizona and New Mexico. Like Colorado and Washington, these neighboring states authorize medical marijuana sales and use among qualified buyers. This allows us to better understand the extent of change in recreational and/or medical marijuana use, if there is one, due to the move from medical marijuana laws to legalization.  In this paper, we discuss the results of our October 2013 baseline survey. This survey describes differences in both recreational and marijuana use across these four states, the proportion of medicinal users who also report using for recreational purposes, and the proportion of medicinal and non-medicinal users who report concurrent use of alcohol.  It lays the groundwork for understanding changes in use associated with marijuana legalization.  Two other surveys – one in April 2014 and another in October 2014 – will enable us to explicitly assess these issues over time as the policy emerges.