We encountered a very rare case of cT0N2M0 small cell lung cancer (SCLC) with Lambert-Eaton myasthenic syndrome (LEMS). A 69-year-old man with a complaint of muscle weakness was admitted to our hospital. Although his chest radiograph on admission showed no abnormal findings, CT scanning detected a mediastinal lymphadenopathy. Also, 2-[18F]-2-fluorodeoxy-D-glucose position emission tomography (FDG-PET) revealed increased accumulation in the same portion in the mediastinum. A diagnosis of LEMS was made from the distinctive electromyogram (EMG) findings (waning and waxing phenomenon in response to low-and high-frequency repetitive stimulation, respectively) in combination with the increased serum level of a P/Q-type anti-voltage-gated calcium channel (VGCC) antibody. Subsequent histopathological diagnosis by mediastinoscopic resection of a paraaortic lymph node was small cell carcinoma. No distant metastasis was detected by MRI of the brain, abdominal CT scan or an FDG-PET. Eight courses of chemotherapy (carboplatin + etoposide) with radiotherapy of the mediastinum (for a total dose of 45 Gy) was performed. A decreased serum level of P/Q-type anti-VGCC antibody titers followed by marked improvement of neurological dysfunction (muscle weakness, gait disturbance and scanning speech) and of an EMG finding (a loss of waning phenomenon) was observed. A close relationship between reduction of the antibody titers and improvement of neurological symptoms after the therapy was noticed. It was suggested that monitoring the level of a P/Q-type anti-VGCC antibody titer in the serum is important for evaluating the efficacy of chemotherapy for LEMS associated with SCLC.