This paper offers a new interpretation of Heidegger's concept of inauthenticity (Uneigentlichkeit) in Being and Time. It breaks from the “conformity interpretation” of inauthenticity, according to which the anonymity of the inauthentic person is due to her conformity to das Man. Rather, it argues that the anonymity of the inauthentic person is due to “existential mania” – a state in which a person denies her death and anxiety, understands her abilities to be limitless, and is perpetually active. It shows how this existential mania – and the anonymity to which it gives rise – is analogous to the mania described by the object relations psychoanalyst Melanie Klein. Finally, drawing on D. W. Winnicott's discussion of mania, it shows how both the inauthentic person's conformity to das Man, and her existential mania, give rise to anonymity.