Patients with damage to the frontal lobes frequently exhibit impaired social behavior, but it is not clear which specific processes are disrupted. The authors investigated the ability to interpret nonverbal emotional expression in patients with lesions involving ventromedial (N=20) or dorsolateral prefrontal cortex (N=9) and in healthy volunteers (N=23). As hypothesized, only patients with ventromedial prefrontal lesions showed impaired task performance relative to normal comparison subjects. These results suggest that deficits in social knowledge, namely difficulty interpreting nonverbal emotional expression, contribute to the aberrant social behavior observed following ventromedial prefrontal cortex lesions.