Stiff person syndrome (SPS) is a rare disease. It causes muscle stiffness in the trunk, limbs, and sometimes the neck. Many cases seem appear to be autoimmune-mediated. A small number of cases may be related to cancer. Workup may include blood tests, imaging, electromyography, nerve conduction velocities, and other tests. Treatment sometimes includes muscle relaxants or immunomodulatory agents. The treatment of SPS is usually overseen by a neuromuscular specialist or (less commonly) a movement disorders specialist.