We report the case of a 50-year-old man with paraneoplastic cerebellar 
degeneration (PCD) and Lambert-Eaton myasthenic syndrome (LEMS) associated with 
primary double lung cancer. He developed acute progressive double vision, 
slurred speech, and gait disturbance. Neurological examination revealed 
diplopia, mild ptosis, bilateral horizontal gaze-evoked nystagmus, and 
cerebellar limb and truncal ataxia. The diffusion image of brain magnetic 
resonance imaging (MRI) revealed no abnormal findings in the cerebellum. On the 
basis of the diagnosis of acute cerebelitis, he was given methylprednisolone 
pulse therapy followed by oral prednisolone, which gradually improved his 
neurological signs and symptoms. The analysis of the possible etiology suggested 
that the PCD was induced by lung cancer, which led to ataxia. A chest computed 
tomography scan revealed mass lesions of irregular shape and unclear margins in 
the upper lobe of the right lung and a small nodule tumor in the upper lobe of 
the left lung. We performed transbronchial needle aspiration and detected the 
bronchioloalveolar carcinoma of the right lung. An electromyogram showed waxing 
phenomenon in the ulnar nerve at high-frequency (50Hz) stimulation. The serum 
levels of anti-P/Q-type voltage-gated calcium channel (VGCC) antibody were 
elavated in the patient. These findings confirmed that the pathogenesis of the 
condition of this patient to be associated with LEMS. His cerebellar symptoms 
were considered to be caused by the PCD, and the diplopia, ptosis, and 
hyporeflexia were attributed to LEMS. We performed upper left lobectomy with 
mediastinal lymphnode dissection via video-assisted thoracoscopic surgery. A 
histological study detected small cell carcinoma. A diagnosis of double primary 
lung cancer was made. Physicians need to be aware that patients may develop PCD 
and LEMS associated with anti-VGCC antibody caused by small cell lung cancer, 
and a mass survey should be conducted and careful examinations performed.