OBJECTIVES: To investigate the autoantibody status of patients with myasthenia gravis (MG) and to evaluate its usefulness for disease classification. DESIGN: Retrospective cohort study of patients with MG, who have autoantibodies to receptors and ion channels expressed at neuromuscular junctions and in muscles that impair neuromuscular transmission. One of the autoantibodies studied was a recently identified, novel, MG-specific autoantibody to a voltage-gated potassium (Kv) channel, Kv1.4. SETTING: Keio University Hospital, Tokyo, and Iwate Medical University Hospital, Morioka. PATIENTS: Two hundred nine patients with MG. MAIN OUTCOME MEASURES: Anti-Kv1.4 antibody was measured by an immunoprecipitation assay with sulfur 35-labeled extract from rhabdomyosarcoma cells. Antititin antibody was detected with a commercially available enzyme-linked immunosorbent assay. RESULTS: Anti-acetylcholine receptor, anti-Kv1.4, and antititin antibodies were detected in 150 (72%), 26 (12%), and 50 (24%) of the 209 patients with MG, respectively. All of the patients who were positive for anti-Kv1.4 or antititin antibody were seropositive for the anti-acetylcholine receptor antibody. They were classified into 4 groups based on their status in regard to 3 MG-related autoantibodies: anti-Kv1.4, antititin, and anti-acetylcholine receptor. Clinical associations were found between anti-Kv1.4 and bulbar involvement, myasthenic crisis, thymoma, and concomitant myocarditis and/or myositis; between antititin and older-onset MG; between anti-acetylcholine receptor alone and younger-onset MG; and between seronegativity and ocular MG. In addition, patients with MG in the anti-Kv1.4 group had more severe manifestations of disease than those in the other 3 groups. CONCLUSION: Classification of patients with MG based on autoantibody status may be useful in defining clinical subsets.