Using microdata for a sample of about 350 workers employed by an Italian public institute, we explain individual absence rates considering both variables that may be related to health conditions and variables that may suggest shirking behaviour. Among these variables we especially focus our attention on the influence produced by the behaviour of randomly assigned peers. In order to handle reflection problems, we combine random assignment with the use of instrumental variables. The proportion of females in the peer group is used as an instrument of peer absence behaviour. From Two-Stage Least Square estimates it emerges that social and group interaction plays an important role in shaping individual absence behaviour.