Our purpose in undertaking the present study was to examine the hypotheses proposed for explaining the frequent comorbidity of bipolar disorder and multiple sclerosis. One hypothesis posits that, when there is comorbidity, MS plays a causal role in psychiatric manifestations. Another suggests that both disorders have a common underlying physiological process that increases the likelihood of their co-occurrence. We examined two adult siblings with comorbidity and their relatives, including three generations of family members with psychiatric morbidity. We found an extensive multigenerational history of bipolar disorder in this family. This history would seem to support the hypothesis of a common underlying brain process (potentially genetically-based) to explain the comorbidity of BD and MS, but cannot clarify whether this comorbidity implies a relationship between the two disorders or merely reflects parallel processes of brain deterioration. We cannot, however, rule out the possibility of a subclinical MS-related process leading to the early manifestation of BD, with MS appearing much later in time, or even a third, undetermined factor, leading to familial comorbidity. Although we have insufficient information to support either hypothesis definitively, we present the familial cases as a springboard for a discussion of dilemmas related to teasing apart MS and BD comorbidity. Further observation of the clinical course of the younger family members, who have not yet shown any neurological signs, over the next few years may elucidate the current picture further.