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Nitrous oxide (N2O) is known to induce cobalamin (vitamin B12, Cbl) deficiency, leading to myeloneuropathies. We describe two patients who present Cbl deficiency after N2O abuse for several months. They complained weakness of both lower limbs and gait disturbance. Their magnetic resonance imaging demonstrated high signal intensities on the dorsal columns of the spinal cord at C2-C5 on T2 weighted images, suggestive of subacute combined degeneration (SCD). Initial electrodiagnostic studies resulted in demyelinating and axonal motor dominant polyneuropathies (PNs). In these cases, Cbl deficiency due to N2O inhalation was suspected as the primary cause for SCD. Cbl deficiency, however, is mainly known to affect sensory nerves, and therefore difficult to account for the motor dominant PNs in our cases. Based on such fact, we suggested that N2O-induced motor dominant PN may occur independently from Cbl deficiency in SCD patients.