Most studies of facial expressivity in patients with focal brain lesions have examined the ability to produce emotional expression in laboratory settings using various experimental paradigms. In this study, we compared facial expressivity in right hemisphere damaged patients (RHD), left hemisphere damaged patients (LHD), and normal controls (NHD) during videotaped semistructured interviews with the patient and spouse in their home. Three research assistants rated 120 10-sec segments of videotape per patient on a 7-point expressivity scale. We found that RHD patients showed reduced facial expressivity in comparison to both LHD and NHD subjects during spontaneous conversation. In particular, RHD patients demonstrated significantly less smiling and laughter than LHD patients and normal controls. These findings were not a general feature of communicative competence in RHD patients as discourse production equaled that of normal controls. These results support the hypothesis that the right hemisphere mediates facial expressivity during spontaneous social interaction. © 1993 by Academic Press, Inc.