A Thirty seven year old female patient was evaluated for complaint of walking difficulty more prominent in right leg that increase whenever she climbed up stairs. Patient was previously evaluated for particularly for polyneuropathy etiology. Protein value was found 313 mg/dl in cerebrospinal fluid (CSF) examination. Monoclonal gammopathy was detected in serum and CSF of patient. IgG value in CSF of patient was 39.0 mg/dl. A mass consistent with medullary infiltration was detected in thoraco-lumbar CT. Patient was operated for lytic lesion and pathologic examination of excisional biopsy material was reported as ‘'plasma cell tumor''. As a result of detailed clinical and electrophysiologic evaluations the patient was diagnosed with paraneoplastic sensory motor mixed type polyneuropathy. Consequently, paraneoplastic processes should be considered in etiology of acute and progressive onset polyneuropaties and if possible, detailed studies for etiology should be performed in these patients.