This is a sixteen-week, randomized, double-blind add-on study of valacyclovir versus placebo in approximately 60 outpatients meeting diagnostic criteria for DSM-IV Bipolar I or II disorder, testing positive for HSV-1 and who have demonstrable cognitive impairment defined as a total score of less than 85 (one standard deviation from the normal range) on the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS). Each patient will be randomized to double-blind treatment with either valacyclovir or placebo for sixteen weeks. All subjects will be maintained on a stable regimen of psychiatric drugs prescribed by their treating psychiatrist. Patients will be evaluated every 2 weeks by the treatment team and mood rating scales will be administered at each visit including the YMRS, PANSS and the MADRS. The RBANS will be administered again at 8 and 16 weeks. Both the treatment team and the patient will remain blinded during the course of the study. Following the active treatment phase, patients will receive treatment as clinically indicated.

Primary Hypothesis:

Valacyclovir will be superior to placebo in reducing cognitive symptoms associated with bipolar disorder in subjects who have been previously infected with Herpes Simplex virus I (HSV-1).

Secondary Hypothesis:

Valacyclovir will be superior to placebo in reducing mood symptoms associated with bipolar disorder in subjects who have been previously infected with HSV-1.