The influence of age at onset, sex, duration of disease and severity degree on the response to therapy have been evaluated in 30 patients with myasthenia gravis treated with prednisone. During the evaluation period (3 months as a maximum) the patients were only treated with prednisone. The evaluation of the response was based on the use of quantified tests of clinical fatigability. Among all the evaluated factors, only the age at onset seemed to have a predictive value of the response to prednisone therapy. The patients older than 40 years and particularly those older than 60 years showed a better response than younger patients.