Is workers' health more sensitive to losses than gains in job security? While loss aversion, whereby losses loom larger than gains, is typically examined in relation to decisions about anticipated outcomes, I first show using a large sample of workers from the European Household Community Panel and value-added models that losses in job security have a larger effect on health than equivalent job security gains. Second, I address endogeneity issues using the 1999 rise in the French Delalande tax as a quasi-natural experiment. It allows evaluating separately the causal impact of exogenous gains and losses in job security on workers' health. Difference-in-differences estimation results confirm that lower job security generates significant and robust losses in self-assessed health. Meanwhile a greater feeling of job security does not translate into a higher level of self-assessed health. These results are in line with the predictions of the model linking job security to health under the hypothesis of loss aversion built in this paper. This article also demonstrates that losses in health induced by lower job security are not transitory.