We examine the evidence that the behavior of clinicians is impacted by the fact that they are being observed by a research team. Data on the quality of care provided by clinicians in Arusha region of Tanzania show a marked fall in quality over time as new patients are consulted. By conducting detailed interviews with patients who consulted both before and after our research team arrived we are able to show strong evidence of the Hawthorne effect. Patient-reported quality is steady before we arrive, rises significantly (by 13 percentage points) at the moment we arrive and then falls steadily thereafter. We show that quality after we arrive begins to look similar to quality before we arrived between the 10th and 15th consultations. Implications for quality measurement and policy are discussed.