The distribution of ABO blood groups and rhesus factor was studied in patients with myasthenia as compared with the control. There was a statistically significant association of the diseases with the rhesus-negative phenotype and that of generalized myasthenia concurrent with thymoma with the B (III) blood group. The examination revealed no other determinants of the significant association with the disease. The values of a disease risk were obtained for persons having myasthenia-associated signs. It is concluded that the Rh-negative phenotype shows a 1.3-fold increase in the risk of the disease as compared with those having Rh-positive persons.