Stiff-person syndrome (SPS) is a progressive neurological disorder characterized by stiffness of the trunk or limb muscles and frequent muscle spasms induced by unexpected visual, auditory, or somatosensory stimuli. It is an incapacitating disorder that leads to recurrent falls and impaired ambulation. The cause of the disease is unknown but an autoimmune pathogenesis is implicated based on its association with other autoimmune diseases and auto-antibodies, specific HLA haplotypes and high titer antibodies against GAD, the rate-limiting enzyme for the synthesis of GABA. Understanding the autoimmune mechanisms of SPS is fundamental to refine the diagnostic criteria and develop specific therapies. The goals of this study are: a) define the natural history of SPS in a homogeneous cohort of patients, b) explore a pathogenetic link between SPS and viral infections based on the known peptide homology between GAD and certain viruses and c) establish GAD-specific T-cell clones and search for candidate antigenic epitopes using synthetic peptide libraries. Collected clinical data will be used to delineate the rate of disease progression and the frequency of association with other autoimmune illnesses, auto-antibodies, or malignancies. It is anticipated that the knowledge acquired from the study will help us understand the mechanism of the disease and design antigen-specific therapeutic strategies. This is an investigative study intended to define the natural history and pathogenesis of SPS. No new therapy will be provided except of standard care.