Fear of pain and its relationship to dental fear was investigated by measuring autonomic reactions (skin conductance and heart rate) in individuals reporting high and low dental fear when in the presence of a cue that threatened the presentation of electric shock (“threat”) or not (“safe”). Acoustic startle probes were also presented during both threat and safe periods, and the reflexive eye blink, the skin conductance response, and cardiac changes to the startle probe measured. All participants reacted with greater defensive reactivity, including potentiated startle blinks, heightened skin conductance, and cardiac deceleration in the context of threat, compared to safe, cues. Individuals reporting high dental fear were significantly more reactive during threat periods, compared to low fear individuals, showing larger blink reflexes and heightened electrodermal activity, as well as heightened autonomic responses to the startle probe itself. Individual differences in defensive reactivity persisted even after participants received a single mild shock halfway through the experiment. The data indicate that threat of shock elicits heightened defensive reactivity in those reporting high dental fear, consistent with the hypothesis that fear of potentially painful events may be a potent mediator of the anxiety involved in anticipated medical and dental treatment.