Abstract BACKGROUND Guillain-Barre syndrome (GBS) is an immune acute polyradiculoneuropathy usually presented as an ascending peripheral paresis with absent tendon reflex and albumino-cytologic dissociation on cerebrospinal fluid (CSF) exam.We describe two oncologic patients with GBS who presented with atypical symptoms. RESULTS The first patient was a 35 years old woman with newly diagnosed grade 3 invasive breast ductal carcinoma. During the sixth courses of neo-adjuvant chemotherapy she complained at back pain and within 24 hours developed progressive severe bilateral proximal paraparesis with a D10 sensory level. Spine MRI demonstrated diffuse leptomeningeal enhancement. Since leptomeningeal spread (LMD) is uncommon at this stage of breast cancer CSF was obtained. CSF sample revealed albumin-cytologic dissociation (4 cells, protein level 340 mg/dl). EMG showed acute demyelinating neuropathy consistent with GBS. Intravenous immunoglobulins (IVIG) therapy was initiated with clinical improvement. The second patient was a 45 years old woman with newly diagnosed T-Cell lymphoma. Before initiating chemotherapy she presented with a rapidly progressive tetraparesis, hyperreflexia and cervical sensory level. Spine MRI was intact. CSF showed 22 cells without atypia, FACS negative, protein level was 74 mg/dl. The patient worsened clinically. A second CSF sample was obtained and demonstrated an albumin-cytologic dissociation (8 cells without atypia, protein level 115 mg/dl) raising the possibility of GBS. EMG showed acute axonal polyneuropathy consistent with AMSAN variant of GBS. The patient was treated with IVIG with partial recovery. CONCLUSION Atypical presentation of GBS could be challenging among oncologic patients and could lead to misdiagnosis and delay in treatment.