A 68-year-old man was admitted to our hospital because of muscle weakness. A complete medical examination led to a diagnosis of small cell lung carcinoma (SCLC) with Lambert-Eaton myasthenic syndrome (LEMS) and the syndrome of inappropriate secretion of antidiuretic hormone (SIADH). Four courses of chemotherapy (carboplatin + etoposide) and one of radiotherapy with a total dose of 45 Gy to the mediastinum were performed and resulted in a partial response in the SCLC. After the second course of chemotherapy, the serum level of antivoltage-gated Ca2+ channel (VGCC) antibody decreased from 190 pg/ml to 120 pg/ml. Marked improvement of the muscle weakness was recognized only after 3 courses of chemotherapy. The patient, who had had difficulty in standing, recovered enough to be able to climb stairs after 4 courses of chemotherapy. Marked improvement of LEMS was achieved by treatment for small cell lung carcinoma.