Brain-derived neurotrophic factor (BDNF) plays a crucial role in activity-dependent synaptic plasticity and learning and memory. The human functional single-nucleotide BDNF rs6265 (Val66Met) polymorphism has been found to be associated with alteration in neural BDNF release and function correlating with altered emotional behavior. Here, we investigated for the first time the hypothesis that this polymorphism in humans modulates the context dependency of conditioned fear responses. Applying a new paradigm examining generalization of cued fear across contexts, seventy participants stratified for BDNF Val66Met polymorphism were guided through two virtual offices (context) in which briefly illuminated blue and yellow lights served as cues. In the fear context, one light (CS+) but not the other light (CS-) was associated with an electric shock (US). In the safety context, both lights were presented too, but no US was delivered. During the test phase, lights were presented again in both learning contexts and in a novel generalization context without any US. All participants showed clear fear conditioning to the CS+ in the fear context as indicated by potentiation of startle responses and reports of fear. No fear reactions were found for the CS+ in the safety context. Importantly, generalization of fear responses indicated by the potentiation of startle response to the CS+ compared to the CS- in the novel context was evident only in the Met-carrying group. These are the first results to provide evidence in humans that BDNF modulates the generalization of fear responses. Such context dependent generalization processes might predispose Met carriers for affective and anxiety disorders.Neuropsychopharmacology accepted article preview online, 19 November 2013. doi:10.1038/npp.2013.320.