Diminished emotional recognition, expression, and responsivity are frequent legacies of traumatic brain injury (TBI) that can have an adverse impact on relationships and psychosocial recovery. However, assessment of emotion responsivity is often difficult because many patients lack insight into their altered personality. To overcome this obstacle, we used a physiological measure of emotion responsivity, the startle reflex, to examine how this can vary according to the affective valence of stimuli by comparing a TBI group with a matched control group. The study also examined whether weaknesses of attention and speed of information processing could account for differences in startle modulation across groups. Sixty-four TBI patients and controls completed the startle reflex procedure. Participants were presented with pictures that differed in affective valence, and measures were taken of eyeblink startle responses to an acoustic probe. Subjective ratings of affect and arousal for each picture were obtained, and TBI patients completed measures of attention and information processing. Results revealed that the TBI group did not show the pattern of startle modulation observed in the control group. Whilst pleasant pictures produced the usual attenuation of the startle response, startle responses to unpleasant pictures were significantly lower in the TBI group than in controls. No significant correlations emerged between startle responses and performance on neuropsychological measures in the TBI group. The TBI group also rated unpleasant pictures as significantly less arousing than did controls. The results provide partial support for a growing body of evidence that has proposed impaired emotion responsivity following TBI.