We aimed to compare reported observations, participation in, and perceptions of unprofessional behaviors across preclinical and clinical medical students using a 23-item questionnaire that asked participants whether they witnessed or participated in the behavior and considered it unprofessional. Overall, 111 preclinical ("year 3") and 104 clinical ("year 4") students responded. For all of the behaviors, significant positive correlations were present between participation and affirmative perceptions. Participation rates for several unprofessional behaviors (14 of 23 items) were higher in the clinical phase. Clinical students more frequently perceived unprofessional behaviors as appropriate (17 of 23 items) compared with preclinical students. In conclusion, both preclinical and clinical medical students in our setting commonly witness unprofessional behaviors. Clinical students participate in and tend to rationalize these behaviors more frequently than preclinical students do.