The weight and histological appearance of the thymus from patients with myasthenia gravis were studied. Nine patients treated with corticosteroids were compared with 10 patients not treated with corticosteroids before thymectomy. The thymuses of corticosteroid-treated patients were found to have significantly increased fat and connective tissue, decreased germinal centers, and poorer corticomedullary differentiation than those of untreated patients. The glandular weight and degree of myoepithelial stroma were the same in both groups. The changes following corticosteroid treatment mimic those following aging or acute stress, with retention of the myoepithelial stroma but depletion of thymic lymphoid elements. In myasthenic patients not undergoing thymectomy, the possibility remains that persistent myoepithelial tissue can adversely affect the prognosis.