We herein report the finding of a 45-year-old woman suffered from Sjögren syndrome with progressive tetraparesis, who later developed systemic muscle atrophy and respiratory failure with a one-year clinical history. Neurological examinations revealed progressive tetraparesis with absent deep tendon reflexes, whereas no upper motor neuron signs were observed. The motor and sensory nerve conduction velocity and sensory nerve action potential (SNAP) were both completely normal, but the prolongation of distal motor latency in the median nerve and a decrease in the compound muscle action potential (CMAP) amplitude were observed. We ascertained that a spontaneous discharge was detected in her upper and lower limbs on electromyography (EMG). Her neurological findings as well as the EMG findings closely correlated with those of motor neuron disease; however, she showed a motor paralytic bladder and also demonstrated a serum antibody reaction with 50 kDa spinal cord protein of the rat. A lumbar MR image showed an increased signal intensity of the cauda equina on a gadolinium-enhanced T1 weighted image. We consider the immune-mediated impairment of the motor nerve associated with Sjögren syndrome to be the cardinal pathogenesis of the present patient, even though treatment with oral corticosteroids did not ameliorate her symptoms.