Bipolar disorder is a major cause of disability and a high risk for suicide. The pathophysiology of the disorder remains largely unknown. Medication choice for bipolar depression patients involves trial and error. Our group reported previously that brain serotonin 1A (5-HT1A) receptor binding measured by positron emission tomography (PET) is higher in bipolar depression. We now investigated whether pretreatment 5-HT1A levels correlates with antidepressant medication outcome. Forty-one medication-free DSM-IV diagnosed, bipolar patients in a major depressive episode had brain PET scans performed using [11C]WAY-100635 and a metabolite corrected arterial input function. The patients then received naturalistic psychopharmacologic treatment as outpatients and a follow up Hamilton Depression Rating Scale (HDRS) after 3 months of treatment. Patients with 24 item HDRS scores less than 10 were considered to have remitted. A linear mixed effects model was used to compare BPF (binding potential, proportional to the total number of available receptors) in 13 brain regions of interest between remitters and nonremitters. Thirty-four patients completed 3 months of treatment and ratings; 9 had remitted. Remitters and nonremitters did not differ in age, sex, or recent medication history with serotonergic medications. Remitters had higher [11C]WAY-100635 BPF across all brain regions compared with nonremitters (P = 0.02). Higher pretreatment brain 5-HT1A receptor binding was associated with remission after 3 months of pharmacological treatment in bipolar depression. Prospective treatment studies are warranted to determine whether this test predicts outcome of specific types of treatment. Synapse 67:773–778, 2013. © 2013 Wiley Periodicals, Inc.