Using the positron emission tomography ligand trans-1,2,3,5,6,10 β-hexahydro-6-[4-(methylthio)phenyl]pyrrolo-[2,1-a]-isoquinoline ([11C](+)− McNeil 5652), which binds to the serotonin transporter, Oquendo et alArticle report that compared with controls, depressed patients with bipolar disorder have 16% to 26% lower binding potential in the amygdala, midbrain, anterior cingulate cortex, hippocampus, putamen, and thalamus. Binding potential is unrelated to serotonin transporter promoter (5-HTTLPR) triallelic genotype. These findings suggest that bipolar depression is associated with extensive serotonergic dysfunction.