Sigma1 receptors were imaged in living human brain by positron emission tomography (PET) using [11C] SA4503. A dynamic 90-min scan and kinetic analysis enabled quantification of receptor density in the brain. The sigma1 receptors were distributed throughout the brain in normal subjects, but decreased in the frontal, temporal, and occipital lobes, cerebellum and thalamus in patients with early Alzheimers disease and in the putamen in patients with Parkinsons disease. In addition, rates of receptor occupancy by the neuroleptic haloperidol and the selective serotonin reuptake inhibitor fluvoxamine were evaluated by [11C]SA4503-PET and found to be high. [11C]SA4503-PET is useful for studying the pathophysiology of neurological and psychiatric disorders such as schizophrenia and for evaluation of the pharmacodynamics of psychiatric drugs.