When listeners hear a speaker become disfluent, they expect the speaker to refer to something new. What is the mechanism underlying this expectation? In a mouse-tracking experiment, listeners sought to identify images that a speaker was describing. Listeners more strongly expected new referents when they heard a speaker say um than when they heard a matched utterance where the um was replaced by noise. This expectation was speaker-specific: it depended on what was new and old for the current speaker, not just on what was new or old for the listener. This finding suggests that listeners treat fillers as collateral signals.