Previous studies have demonstrated a link between language behaviours and deception; however, questions remain about the role of specific linguistic cues, especially in real-life high-stakes lies. This study investigated use of the so-called filler, 'um', in externally verifiable truthful versus deceptive speech of a convicted murderer. The data revealed significantly fewer instances of 'um' in deceptive speech. These results are in line with our recent study of 'um' in laboratory elicited low-stakes lies (Arciuli, Mallard, & Villar, 2010). Rather than constituting a filled pause or speech disfluency, 'um' may have a lexical status similar to other English words and may be under the strategic control of the speaker. In an attempt to successfully deceive, humans may alter their speech, perhaps in order to avoid certain language behaviours that they think might give them away.