This study aimed to determine the prevalence and sources of stress among Thai medical students. The questionnaires,which consisted of the Thai Stress Test (TST) and questions asking about sources of stress, were sent to all medical students in the Faculty of Medicine, Ramathibodi Hospital, Thailand. A total of 686 students participated. The results showed that about 61.4% of students had some degree of stress. Seventeen students (2.4%) reported a high level of stress. The prevalence of stress is highest among third-year medical students. Academic problems were found to be a major cause of stress among all students. The most prevalent source of academic stress was the test/exam. Other sources of stress in medical school and their relationships are also discussed. The findings can help medical teachers understand more about stress among their students and guide the way to improvement in an academic context, which is important for student achievement.