Microscopic thymoma was first described in 1976 and remains in the latest WHO classification (2004). It defines an epithelial proliferation, with <1 mm in diameter, usually multifocal, that preferentially occurs in patients suffering from myasthenia gravis without a macroscopically evident tumour. A series of 87 thymectomies realised in myasthenic patients between 1992 and 2005 was reviewed. We describe three cases of microscopic thymoma: two male and one female with an average age of 41.6 years. All patients suffered from myasthenia gravis positive for acetylcholine receptor antibodies without any tumour detectable on chest tomodensitometric examination. Thymectomy was performed in all cases, enlarged to the mediastinal fat in one. Microscopic examination revealed three type-A microscopic thymoma, one multifocal, associated with lymphoid hyperplasia in two cases. After surgery, all patients had clinically improved. In summary, chest computed tomography is unable to detect microscopic thymoma. Histological examination of the entire specimen is indicated after thymectomy in myasthenic patients even if the thymus appears normal on radiographic examination. The role of microscopic thymoma as a predictor of clinical thymoma has not been proven.