Twin pairs with one or two myasthenic patients were selected from patients with neuromuscular pathology. 18 couples of twins were included in final selection (9 monozygotic and 9 dizygotic pairs). 4 pairs of monozygotic twins were concordant to myasthenia (MA). All dizygotic pairs were discordant to this disease. Matched concordance of monozygotic twins was 44%. Penetration of pathological gene was 61% assuming the hypothesis about monogenic heredity of MA. The coefficient of heredity was 44%. The conclusion was made about important, but not absolute role of hereditary factors in development of MA. It was necessary the presence of combination of both genetic and environmental factors for MA development.