We investigate a case of nodular hyperplasia of the thymic epithelium which was incidentally, microscopically discovered. Macroscopically there was no sign of tumor and the thymus was surgically removed for the therapy of the clinical symptoms of the myasthenia gravis worsened in two years of evolution. Histological in a general appearance of an involuted thymic tissue, a small nodular epithelial proliferation was identified. The epithelial proliferation was classified as A-type in the WHO histological classification of the thymic epithelial tumors. Generally, these microscopic thymomas range from 0.2 mm to 0.4 mm in size that corresponds to our finding that measured 0.25/0.35 mm. This lesion was singular; on additional sections examined, we did not find other areas. Even so, there is a tight connection between the myasthenia gravis, thymomas and these microscopic thymomas, the development of a thymoma from this lesion has not been proven.