Genetically proved monozygotic female twins in whom myasthenia gravis developed in their 20s initially had their disease well controlled with anticholinesterase medication. Because of increasing resistance to medication, twin 1 had her thymus removed, after which the symptoms decreased. Predicated on the improvement in her sister and the need for increasing medication to allay symptoms, and after proof of monozygosity, twin 2 also underwent thymectomy, with subsequent symptomatic improvement. The twins were followed up nine and six years after thymectomy, respectively. Monozygotic twins with myasthenia gravis are generally young women, with onset of disease in one occurring within one to three years of the other. Reports of only one affected twin may be misleading because of inadequate documentation of monozygosity, absence of long-term observation, or both. Serial investigations of the "uninvolved" twin in a monozygous pair and proof of monozygosity should be obtained to aid in early diagnosis and treatment of this illness, as well as to study the pathogenesis of myasthenia gravis prior to symptom onset.