A 72-year-old man was admitted to our hospital with complaints of dry mouth, muscle weakness of the lower limbs and gait disturbance. The patient had dry mouth, dry eyes, positive anti-SS-B antibody and salivary gland inflammation. Sjögren's syndrome was diagnosed. Since muscle weakness of the lower limbs and gait disturbance were not compatible with Sjögren's syndrome, we considered the possibility of paraneoplastic syndrome. Serum levels of CEA, NSE and ProGRP were elevated. Chest roentgenogram and CT showed a nodular lesion in the right upper lobe of the lung and swollen lymph nodes in the hilum and mediastinum. Small cell lung cancer was diagnosed by bronchoscopy. Anti-P/Q-type Ca2+ channel antibody was positive. Electromyogram showed a reduction in the amplitude of the evoked muscle action potential response after slow repetitive stimulation and did not show a reduction after rapid repetitive stimulation. Based on these findings, we made a diagnosis of Lambert-Eaton myasthenic syndrome (LEMS). Concurrent chemoradiotherapy induced an improvement of muscle weakness of the lower limbs. LEMS is frequently associated with a malignant tumor and an autoimmune disorder. We thought that in this patient, the presentation of LEMS was apparent because he had both Sjögren's syndrome and small cell lung cancer.