We study the effect of obesity on employment, using rich data from the British National Child Development Study (NCDS). The results show a significant negative association between obesity and employment even after controlling for a rich set of demographic, socioeconomic, environmental and behavioral variables. In order to account for the endogeneity of obesity, we use and assess instruments introduced by Cawley (2004); the obesity status of biological relatives. Using parental obesity as an instrument, we show that the association between obesity and employment is no longer significant. Similar results are obtained in a model of first differences. We provide a number of different checks on the instruments, by exploiting the richness of the NCDS data. The results show mixed evidence regarding the validity of the instruments. © 2010 Elsevier B.V.