This study was performed to evaluate the titer of serum acetylcholine receptor antibody (AChR-Ab), the correlation between AChR-Ab titer and clinical state, clinical response to thymectomy and histopathologic finding of thymus in myasthenia gravis. Twenty-seven patients with various clinical grades of myasthenia gravis and twenty-three norrnal controls were included in this study. Mean AChR-Ab titers were 4.21+4.27nM in myasthenia gravis and 0.05+0.06nM in control group(p<0.05). Mean AChR-Ab titers of each clinical grade were 0.80+1.67nM in grade I, 5.05+3.42nN in grade Iia, 8.37+4.50nM in grade Iib, 6.67nM in grade m and 10.89nM in grade IV. There were significant correlation between clinical grade and level of AChR-AB titer. There were no correlation between degree of clinical improvement and changes of serum AChR-Ab titer after thymectomy in myasthenia gravis. There were also no correlation between level of AChR-Ab titers and histopathologic findings of thymus.