Efficient social interactions require taking into account other people’s mental states such as their beliefs, intentions or emotions. Recent studies have shown that in some social situations at least, we do spontaneously take into account others’ mental states. The extent to which we have dedicated brain areas for such spontaneous perspective taking is however still unclear. Here, we report two brain-damaged patients whose common lesions were almost exclusively in the left posterior temporoparietal junction (TPJp) and who both showed the same striking and distinctive theory of mind (ToM) deficit. More specifically, they had an inability to take into account someone else’s belief unless they were explicitly instructed to tell what that other person thinks or what that person will do. These patients offer a unique insight into the causal link between a specific subregion of the TPJ and a specific cognitive facet of ToM.