The authors describe the first four years (1995-1998) in which the University of California, San Francisco School of Medicine operated an evaluation system to monitor students' professional behaviors longitudinally through their clinical rotations. The goals of this system are to help "turn around" students found to have behaved unprofessionally, to demonstrate the priority placed by the school on the attainment of professional behavior, and to give the school "muscle" to deal with issues of professionalism. A student whose professional skills are rated less than solid at the end of the clerkship receives a "physicianship report" of unprofessional behavior. If the student receives such a report from two or more clerkships, he or she is placed on academic probation that can lead to dismissal even if passing grades are attained in all rotations. Counseling services and mentoring by faculty are provided to such students to improve their professional behaviors. From 1995 to 1998, 29 reports of unprofessional behavior on the part of 24 students were submitted to the dean's office; five students received two reports. The clerkship that submitted the most reports was obstetrics-gynecology. The most common complaint for the five students who received two reports was a poor relationship with the health care team. Four of these students had their difficulties cited in their dean's letters and went on to residency; the fifth voluntarily withdrew from medical school. The authors describe the students' and faculty members' responses to the system, discus lessons learned, difficulties, and continuing issues, review future plans (e.g., the system will be expanded to the first two years of medical school), and reflect on dealing with issues of professionalism in medical school and the importance of a longitudinal (i.e., not course-by-course) approach to monitoring students' behaviors. The authors plan to compare the long-range performances of students identified by the evaluation system with those of their classmates.