Bipolar disorder can emerge in the context of organic brain pathology. In the case presented, long-standing hydrocephalus was diagnosed in a man with relatively late-onset bipolar illness who presented initially with somewhat atypical, treatment-resistant depressive symptoms. Hypomania, followed by a rapid-cycling bipolar course, subsequently developed. This report reviews the association between bipolar disorder and hydrocephalus, and examines possible neurobiological mechanisms implicated in both conditions.