This study determined rates and correlates of stress, depressive symptoms/dysphoria, and suicidal thoughts among US medical students. Data were obtained from a large-scale survey conducted at 16 representative medical schools. Students completed questionnaires during first-year orientation (n= 1846), at the time of transition to clinical rotations (n= 1630), and during the fourth year (n= 1469). Students entering the wards reported greater stress, more bad mental health days, and greater depressive feelings than first-year students, with fourth-year students reporting intermediate stress levels. The number of days of bad mental health in the past month, stress experienced in the past 12 months, and perceptions about the medical school's system for coping were independently associated with suicidal thoughts, which were reported by 9% of fourth-year students. Medical schools should undertake efforts to assist students' coping because a substantial proportion of students experience meaningful levels of stress, depressive symptoms (especially around the time of transitioning to clinical care rotations), and suicidal thoughts.