'BACKGROUND: Deficits in social cognition, e.g. theory of mind (ToM) represent core characteristics involved in the etiology of psychopathological symptoms and an important predictor of social competence. In bipolar affective disorder, evidence for ToM deficits is scant, although brain imaging studies and impairments in non-social neurocognitive domains indicate deficits of prefrontal functions. METHODS: Twenty-nine euthymic patients with bipolar affective disorder and 29 matched healthy controls were examined with the ''Movie for the Assessment of Social Cognition'' (MASC). Target parameters comprised 1) ''cognitive'' and ''emotional'' ToM scores, 2) qualitative analysis of errors brought about by ''undermentalizing'' or ''overmentalizing'' strategies and 3) non-social inferencing as a control factor. RESULTS: Patients compared to controls scored significantly lower for ''cognitive'' (F=9.417, df=1, p=0.003) but not for ''emotional'' ToM. Bipolar patients showed significantly higher ''undermentalizing'' (F=4.830, df=1, p=0.032) but not ''overmentalizing'' scores. A significant correlation (controlled for age) between the number of (hypo)manic episodes and ''undermentalizing'' (r=0.527, p=0.030) as well as ''emotional'' ToM (r=-0.546, p=0.023) was observed. LIMITATIONS: Sample size did not allow for the analysis of medication effects. DISCUSSION: This is the first study of ToM in euthymic bipolar patients with a realistic video-based examination. Independently from basic cognitive dysfunctions, patients displayed a pronounced deficit in the cognitive domain of ToM with preserved emotional mentalizing abilities. The correlation with the number of manic episodes may indicate an increase of the deficit with disease progression. It can be assumed that social cognition reflects an important dimension of the persisting cognitive deficits in bipolar disorder with possible impact on disease outcome.'