Objective: This study sought to examine how specific substance-use behavior, including nonmedical prescription stimulant (NPS) use, among U.S. medical students correlates with their attitudes and beliefs toward professionalism. Method: An anonymous survey was distributed to all medical students at a private medical university (46% response rate). Participants were asked to report alcohol and marijuana use patterns, NPS use, stress levels, and history of suicidal ideation. Results: Over one-third of medical students reported excessive drinking during the past month, and 5% reported NPS use during the past year. Students who endorsed such behavior were significantly less likely to view it as unprofessional and warranting intervention. A large number of students seemed unfamiliar with how to help a classmate with an NPS use problem. Conclusions: Medical students' substance use behaviors appear to influence attitudes and beliefs toward professional issues regarding substance use. (Contains 2 tables.)