Background: The number of visitors to Tibet has been increasing recently and reached to more than six million in 2010. Many of the tourists are not aware of the risk of altitude sickness. The most common altitude sickness to high altitude visitors is acute mountain sickness (AMS), which is unpleasant and may progress to the life threatening condition of high altitude cerebral oedema (HACE) and high altitude pulmonary oedema (HAPE). These problems are of public concern as they have health implications for the health authorities and economic implications for tourism industry in Tibet. Yet, so far no research on acute mountain sickness among tourists has been conducted in Lhasa. Aim: The study aims to estimate AMS prevalence among tourists and its various subgroups in Lhasa. Methods: A cross-sectional study was conducted among 2385 tourists in Lhasa who were above the age of 15 years. The participants were recruited in the randomly selected hotels in Lhasa during the time of the study was carried out between the 2nd of June and the 31st of October 2010. The Lake Louise Score System (LLSS) was used to assess AMS. Both English and Chinese version of LLSS-based questionnaire was used for data collection. Results: AMS prevalence was 50.8% in total sample and 44% in subset two hotels¡¯ sample. The AMS prevalence was higher among participants without previous experience of high altitude than with the experience and, in people aged between 20 and 60 as compared with those younger or older and it was more often found in men than in women. The AMS was more common among participants, who arrived in Lhasa by air, who took medication for preventing AMS before coming to Tibet and pain-relieving medication after they arrived in Tibet, and among those who have had previous experience AMS symptoms.Conclusion: The estimated prevalence of AMS among tourists in Lhasa is high and lies between 44%¨C50.8% using the LLSS AMS diagnostic standards (headache plus one other symptoms and LLSS¡Ý 3). The prevalence of AMS was significantly higher in people who rapidly ascend to Tibet, middle aged, experienced at high altitude and previous AMS symptoms.