Stiff person syndrome is a rare disease, with an estimated prevalence of 1–2 cases per 1,000,000 individuals. In most patients, symptoms result from GABAergic system dysfunction caused by antibodies against glutamic acid decarboxylase. Antiamphiphysin antibodies are detected less often. In those cases, paraneoplastic origin must be excluded. The hallmark symptoms of the disease include progressive rigidity and spasms of the extremities, paravertebral and abdominal muscles. Rigidity of the torso causes hyperlordosis. These symptoms lead to severe disability. Common comorbidities include diabetes, thyroid disorders, and psychic disturbances. This paper describes the case of a young male who experienced progressive rigidity and paresis of the limbs over several years before the diagnosis of stiff person syndrome was finally established. The therapeutic process is also presented. Although stiff person syndrome is a rare disease, it must be taken into account in the differential diagnosis of the patients with limb rigidity and paresis of uncertain aetiology.