In the study described below, 97 myasthenia gravis (MG) patients have been evaluated for antibody reactivity against muscle myosin. With few exceptions, we could clearly observe a positive correlation between the presence of circulating serum anti-myosin antibodies and disease severity. Unlike some studies of anti-striational antibodies in MG patients, no association with thymoma was apparent. Moreover, no relationships were revealed between the severity of disease symptoms demonstrated and absolute anti-myosin antibody titers or prominent IgG subclasses. Therefore, we suggest that expression of myosin reactivity may be one useful parameter for predicting disease severity in those patients, but it is not prerequisite for induction of MG.