Most of the patients with Lambert-Eaton myasthenic syndrome (LEMS) are over 40 years of age at onset and have a malignant neoplasm, especially pulmonary small cell carcinoma. Juvenile cases of LEMS are rare and have not been reported in Japan. We report a 21-year-old women whose onset of LEMS was at 17 years of age. In the early stage of the disease, she had limb muscle weakness and was diagnosed has having seronegative myasthenia gravis (MG) based on decrementing response in low frequency repetitive nerve stimulation, positive edrophonium test, and negative serum anti-acetylcholin receptor antibodies. Serum anti-voltage-dependent-calcium-channel (VGCC) antibody was negative with an available assay system. She was thymectomized and received anti-cholinesterases, corticosteroids, and cyclophosphamide, but their therapeutic effects were not evident. Two years later, she was confined to bed due to sever emaciation (Her body weight reduced from 47 Kg to 27 Kg.), bulbar palsy, and limb muscle weakness. She needed respiratory support and tube feeding. Type II fiber atrophy, which is non-specific, but is a common finding in LEMS, was seen in the biopsied muscle of the patient. On the other hand, postsynaptic folds and clefts of all seven motor end-plates in the specimen were normal, which is against the diagnosis of MG. Recently, she was found to have anti-VGCC antibody with an improved assay system. Despite vigorous examination, no malignancy or other autoimmune disorders have been found. 3,4-Diaminopyridine was effective to improve limb muscle powers, but she is still bedridden.