Physicians function as clinicians, teachers, and role models within the clinical environment. Negative learning environments have been shown to be due to many factors, including the presence of unprofessional behaviors among clinical teachers. Reliable and valid assessments of clinical teacher performance, including professional behaviors, may provide a foundation for evidence-based feedback to clinical teachers, enable targeted remediation or recognition, and help to improve the learning environment. However, few tools exist for the evaluation of clinical teachers that include a focus on both professional and clinical teaching behaviors. The Professionalism Assessment of Clinical Teachers (PACT) was developed and implemented at one Canadian institution and was assessed for evidence of reliability and validity. Following each clerkship rotation, students in the 2009-2010 third-year undergraduate clerkship cohort (n = 178) anonymously evaluated a minimum of two clinical teachers using the PACT. 4,715 forms on 567 faculty members were completed. Reliability, validity, and free text comments (present in 45% of the forms) were examined. An average of 8.6 PACT forms were completed per faculty (range 1-60), with a reliability of 0.31 for 2.9 forms (harmonic mean); 12 forms were necessary for a reliability of 0.65. Global evaluations of teachers aligned with ratings of free-text comments (r = 0.77, p < 0.001). Comment length related negatively with overall rating (r = -0.19, p < 0.001). Mean performance related negatively with variability of performance (r = -0.72, p < 0.001), although this may be related to a ceiling effect. Most faculty members were rated highly; however "provided constructive feedback" was the least well-rated item. Respectful interactions with students appeared to be the most influential item in the global rating of faculty performance. The PACT is a moderately reliable tool for the assessment of professional behaviors of clinical teachers, with evidence supporting its validity.