- The search for psychological markers or for psychological endophenotypes for bipolar affective disorder has been frustrating, and the study of neuropsychological and neurocognitive functioning may be useful in this search. This article presents the results of a study comparing Rorschach protocols from a sample of adult euthymic bipolar patients (N = 18) and matched healthy controls (N = 8). Bipolar patients showed a higher proportion of immature responses and more instances of thought disorder; patients also showed significantly more severe thought disorder. These findings are discussed in the context of 2 related previous studies. We suggest that our modest series of studies using the Rorschach Inkblot Test provides preliminary evidence that certain variables—especially the measures of thought disorder but possibly also the lack of emotional responses under cognitive “control” and the excessive proportion of immature content responses—may represent a possible endophenotype of bipolar disorder.