Myasthenia gravis in identical twins.

The literature records myasthenia gravis in five sets of monozygous twins; we report another pair in which monozygosity was determined by blood group analysis, HLA typing, and mixed lymphocyte culture. Acetylcholine receptor antibodies were strongly positive in both twins. Poor response to anticholinergic medication and thymectomy necessitates low-dose daily maintenance prednisone for a normal life-style. The probability of myasthenia developing in the unaffected monozygotic twin is highest soon after diagnosis in the proband and an unaffected twin should be followed indefinitely.