Thymic cysts were diagnosed by thoracotomy in two women with leprosy, both 64 years old, within 12 months. Both were asymptomatic and an abnormal mediastinal shadow on an annual screening chest radiograph was a clue to the diagnosis in both cases. According to retrospective studies it took at least six or seven years for the thymic cysts to become large enough to be detected. Real-time echography was effective in the diagnosis of a cystic lesion in one case. Thymic cysts are important cystic lesions rarely found in the anterior mediastinum. A resective operation is thought to be indispensable to differentiate them from thymomas accompanied by large cystic degeneration and from thymic cysts accompanied by malignancies.