Lying is a deliberate attempt to transmit messages that mislead others. Here, we examined the frequency of use of the so-called filler word um during lying versus truth-telling in low-stakes laboratory-elicited lies (Study 1) and also in high-stakes real-life lies (Study 2). Results from a within-subjects false opinion paradigm showed that instances of um occur less frequently during lying compared to truth-telling. Converging evidence was provided upon examining the lies of a convicted murderer. These results contribute to our understanding of linguistic markers of deception behaviour. More generally, they assist in our understanding of the role of utterances such as um in communication. Utterances such as um may not be accurately conceptualised as filled pauses/hesitations or speech disfluencies/errors whose increased usage coincides with increased cognitive load or increased arousal. Rather, they may carry a lexical status similar to interjections and form an important part of authentic, natural communication - that is somewhat lacking during lying.