BACKGROUND: Lambert-Eton myasthenic syndrome(LEMS) is characterized by the clinical triad of muscle weakness, hyporeflexia, and autonomic dysfunction. In contrast to myasthenia gravis.LEMS is not commonly associated with respiratory failure. Any case of respiratory failure in LEMS has not been reported in Korea. CASE DESCRIPTION: The first case is a 61-tear-old male complained of proximal muscle weakness and dysarthria associated with severe dry mouth for 3 month and developed respiratory failure. The second case is a 65-tear-old male who began complaining of muscle weakness, weight loss, dry mouth, and recurrent respiratory difficulty for 14 months. Repetitive nerve stimulation(RNS) test for ulnar nerve showed CMAP with low amplitude, a significant decremental response at the low rate stimulation and a marked incremental response at the high rate stimulation in the left abductor digiti muscles in both cases. The first case had small cell carcinoma of the lung, but the second case presented with respiratory failure in the absence of malignancy. CONCLUSION:We experienced two cases of LEMS presenting respiratory failure. To our knowledge, this is the first report of respiratory failure in LEMS in Korea.