The authors report a 63-year-old man with a history of brief isolated manic episodes who became persistently hypomanic after a small right thalamic infarct. Detailed behavioral and neuropsychologic assessment were performed 18 months after the stroke and revealed a prosopoaffective agnosia as the foremost cognitive disorder, i.e., an impairment in the identification of emotional facial expressions with preserved discrimination of facial identity. Difficulties in reasoning on humorous material and other signs of mild right hemisphere dysfunction were present, but other perceptual, frontal and abstract-reasoning cognitive functions were unimpaired. Prosopoaffective agnosia has not been reported previously in thalamic lesions or in primary or secondary mania. The authors discuss the hypothetical relationships between a right hemisphere deficit in processing emotions and relapsing of the patient's hypomanic behavior.