The objective of our study is to compare the diagnostic significance of mediastinal CT and (201)Tl-single-photon emission computed tomography (SPECT) with thymic histology in patients with myasthenia gravis (MG). The subjects were 30 patients with MG who were scheduled to receive thymectomy. They did not receive immunosuppressive therapy. The mediastinal CT and (201)Tl-SPECT were performed before the thymectomy. As a consequence, 9 patients had thymoma, 11 had lymphoid follicular hyperplasia (LFH), and 10 had normal thymus on histologic examination after thymectomy. Retrospectively, CT diagnosed the histology of all 9 patients with thymoma, and 5 of 11 with LFH. (201)Tl-SPECT could detect abnormal accumulations in only 6 of 9 cases of thymoma, and 6 of 11 cases of LFH. (201)Tl-SPECT could not distinguish thymoma from LFH. We conclude that a CT study is recommended as an essential study for the detection of thymic abnormalities in pre-operative patients with MG.