Medical tourism – patients travelling specifically to access health services abroad – is a growing phenomenon, but one that is under-researched. This chapter presents findings of the first ever study examining medical tourism based on individual patient data, and the most comprehensive analysis to date of the size, shape and impact of medical tourism on the health system and economy. It focuses on Thailand, one of the world’s foremost destinations for medical tourism. A cross-sectional survey of medical tourists in five private Thai hospitals was conducted, comprising 911 913 patient records, as well as a patient survey of 293 medical tourists. In addition, 15 hospital executives and 28 service providers in four private hospitals were interviewed. Findings show previous estimates suggesting over 1 million tourists per year were inflated. Medical tourists in Thailand are non-homogenous. The majority of them are likely to be opportunistic tourists who travel to Thailand with other purposes combining medical services. Most patients travel from within the region. They and their companions contribute to the Thai economy in terms of medical and tourism spending. This research identified no negative consequences for the health system.