Background: Teaching medical students in the ambulatory setting may influence the duration or number of patients per clinic. Purpose: To directly measure the time required to teach medical students in an outpatient clinic and to determine if there was a difference in activities performed by faculty when a student was present in the clinic. Methods: In this prospective, nonrandomized study, 83 clinic sessions were analyzed; 50 without a 3rd-year internal medicine clerkship student and 33 with a student. Seven 3rd-year internal medicine clerkship students and 7 general internists participated. The 7 general internists had both clinic sessions with and clinic sessions without a student during our study period. For every clinic session, physicians recorded duration, number of patients, presence of a student, and teaching activities. In a subset of 23 clinics (28%), ancillary staff independently recorded clinic duration and number of patients seen. To address time added to a clinic session by a student, we compared clinic sessions with versus clinic sessions without a student for each participating physician. Multiple linear regression was used for analysis. Results: Having a student added 32.3 min to a clinic session (p < .001). Clinic duration recorded by ancillary staff did not differ from duration recorded by physicians (p = .74), and the durations were well correlated (r = .81). Regarding additional activities, physicians were more likely to discuss patients with house staff when students were present, but other nonteaching physician activities did not change. Conclusions: In this study, teaching a 3rd-year medical student in an internal medicine outpatient clinic required 32.3 extra min per clinic. Clerkship directors and clinic administrators should be aware of the extra time required to teach and be prepared to expect an impact on clinic productivity.