Lambert Eaton myasthenic syndrome(LEMS) is a paraneoplastic syndrome caused by defects in the secretion of acetylcholine from the presynaptic membrane of nerve terminals and is strongly associated with small cell lung carcinoma. The pathogenesis of LEMS is the destruction of voltage gated calcium channels by an autoimmune process resulting in clinical manifestations consisting of lower extremity weakness, decreased deep tendon reflexes and autonomic dysfunctions. The diagnosis can be confirmed by the characteristic clinical features and repetitive nerve stimulation. The neurological symptoms and signs of LEMS may manifest themselves months before the clinical manifestation of the underlying malignancy. Therefore early diagnosis and treatment of the primary malignancy may become possible through the diagnosis of this rare paraneoplastic syndrome. We report a case of a patient diagnosed with LEMS who upon further evaluation for an underlying malignancy was found to have a 0.2cm sized nodular and infiltrative mass lesion at the bifurcation of the left apicoposterior segmental and anterior segmental bronchi by bronchoscopy. Although repeated bronchoscopic biopsies of the lesion was not able to disclose malignancy, under strong clinical suspicion left upper lobectomy was performed and subsequently the diagnosis of small cell carcinoma of the lung was confirmed. Muscle weakness began to improve starting from a week after the surgery, then reached a plateau 2 weeks later. Muscle weakness improved further after the trial of anticancer chemotherapy.