Sera from patients with myasthenia gravis (MG) were screened for autoantibodies to skeletal muscle-specific antigens by immunoprecipitation assay, using rhabdomyosarcoma and leukemia cell lines. Eleven of 61 MG sera immunoprecipitated a rhabdomyosarcoma-specific 70-kDa protein, which was identified as the voltage-gated K+ channel 1.4 (Kv1.4). This antibody specificity was not detected in 30 patients with polymyositis/dermatomyositis, 9 with thymoma alone, or 30 healthy controls. Clinical features associated with anti-Kv1.4 antibody included bulbar involvement, myasthenic crisis, thymoma, myocarditis, and QT prolongation on electrocardiogram. These findings suggest that anti-Kv1.4 antibody is a novel autoantibody associated with a severe MG subset and thymoma.