Twenty cases of macroscopically non-neoplastic thymuses obtained from patients with myasthenia gravis have been studied histologically. Seven cases were characterized by lymphoid follicular hyperplasia and 13 by involutional changes of variable degree. In three cases (15%), one with lymphoid follicular hyperplasia and two with involutional changes, multiple microscopic epithelial lesions, 0.2-0.4 mm in diameter and consistent with foci of microscopic thymoma, were observed. Most of them were related to the thymic cortex and one, displaying a different histological pattern, was located in a medullary area. These observations provide morphological evidence of a possible multifocal origin of thymoma from distinct epithelial clones present in the different topographic areas of the human thymus.