Informal payments—payments made from patients to health personnel in excess of official fees—are widespread in low-income countries. It is not obvious how such payments affect health worker effort. On the one hand, one could argue that because informal payments resemble formal pay for performance schemes, they will incite higher effort in the health sector. On the other hand, health personnel may strategically adjust their base effort downwards to maximise patients' willingness to pay informally for extra services. To explore the relationship between informal payments and health worker effort, we use a unique data set from Tanzania with over 2000 observations on the performance of 156 health workers. Patient data on informal payments are used to assess the likelihood that a particular health worker accepts informal payment. We find that health workers who likely accept payments do not exert higher average effort. They do however have a higher variability in the effort they exert to different patients. These health workers are also less sensitive to the medical condition of the patient. A likely explanation for these findings is that health workers engage in rent seeking and lower baseline effort to induce patients to pay. Copyright © 2012 John Wiley & Sons, Ltd.