Introduction: Paraneoplastic disorders exert their effect through immunological mechanisms and not by direct invasion of the tumor; in this way tumors located outside the nervous system induce the production of antibodies against antigens: on the neu- ronal surface, intracellular or synaptic and affect distant sites from its location to either the central or peripheral nervous system, thus producing various clinical manifestations. One of these syndromes is the Stiff Person syndrome, which, apart from being a rare entity, occurs as a paraneoplastic syndrome in only 5% of cases.

Clinical Case: A 72-year-old male patient, a 2-month evolution that began with painful muscle spasms in the dorsal and lumbar region, evolved to rigidity with posture in axial and appendicular hyperextension, pseudospastic gait, hyperekplexia; due to the suspicion of Stiff Person syndrome a diagnostic work-up was begun finding high levels of Ab anti GAD65 and later a Non- Hodgkin’s Lymphoma. The patient received immunotherapy and cancer treatment.

Conclusion: Paraneoplastic syndromes must be identified according to the clinical phenotype and the determination of an- tineuronal antibodies. High suspicion and rapid diagnosis are necessary to achieve timely treatment and avoid permanent injury.