To investigate the magnitude and characteristics of medical tourism in Thailand and the impact of such tourism on the Thai health system and economy. In 2010, we checked the records of all visits to five private hospitals that are estimated to cover 63% of all foreign patients. We reviewed hospital records of foreign patients and obtained data on their countries of origin, diagnoses and interventions. We surveyed 293 medical tourists to collect demographic characteristics and information on their expenditure and travelling companions. To help understand the impact of medical tourism on the Thai health system, we also interviewed 15 hospital executives and 28 service providers from the private hospitals. We obtained 911,913 records of hospital visits, of which 324,906 came from 104,830 medical tourists. We estimated that there were 167,000 medical tourists in Thailand in 2010. Of the medical tourists who attended our study hospitals, 67,987 (64.8%) came from the eastern Mediterranean region or Asia and 109,509 (34%) of them were treated for simple and uncomplicated conditions - i.e. general check-ups and medical consultations. The mean self-reported non-medical expenditure was 2750 United States dollars. According to the hospital staff interviewed, medical tourism in 2010 brought benefits to - and apparently had no negative impacts on - the Thai health system and economy. We estimate that the total number of medical tourists visiting Thailand is about 10% of previous national government estimates of 1.2 million. Such tourists appear to bring economic benefits to Thailand and to have negligible effects on the health system.