We study the presence of other-regarding preferences in the workplace by exploit-ing a randomized experiment that changed the monitoring of workers’ health during sick leave.We show that workers’ response to an increase in co-worker shirking, induced by the experi-ment, is much stronger than the response to a decrease in co-worker shirking. The asymmetricspillover effects are consistent with evidence of fairness concerns documented in laboratoryexperiments. Moreover, we find that the spillover effect is driven by workers with highly flexibleand autonomous jobs, suggesting that co-worker monitoring may be at least as important asformal monitoring in alleviating shirking.