The authors review their experience with magnetic resonance imaging (MRI) of the thymus and discuss the appearance of thymic epithelial tumors where MRI is clinically useful. Detailed descriptions of MRI findings in benign thymomas, invasive thymomas, and thymic carcinomas are provided. Most benign (noninvasive) thymomas appear with a slightly higher signal intensity than that of muscle on T1-weighted images. On T2-weighted images, thymomas have an increase in signal intensity on both T1- and T2-weighted images. Neither fibrous septa nor lobulated internal architecture can be detected on MRI. Gd-DTPA-enhanced MR images show homogeneous enhancement. On the other hand, invasive thymomas show the same signal intensity as benign thymomas, both on T1- and T2-weighted images. However, invasive thymomas appear inhomogeneous in signal intensity on T2-weighted images. T2-weighted images also show a lobulated border, fibrous septa, and lobulated internal architecture, characteristic of most invasive thymomas. Irregularity of tumor margins indicating invasion into surrounding structures is noted in some cases of invasive thymomas. Exceptionally minute thymomas (< 1 cm in diameter) show a different signal intensity on MRI as compared to those of usual thymomas: both T1- and T2-weighted MR images show a low signal intensity mass with irregular or unclear borders. Histopathologically, these minute thymomas contain numerous tiny cysts and/or abundant collagenous tissues. Generally, thymic carcinomas, except carcinoid tumors, appear with a relatively low signal intensity on T1- and T2-weighted MR images in comparison to those of thymomas. In particular, well-differentiated squamous cell carcinomas appear with a low signal intensity on both T1- and T2-weighted images. Abundant collagenous tissue may be a causative factor for the low signal intensity on T2-weighted MR images. Thymic carcinomas appear slightly inhomogeneous on both T1- and T2-weighted images. Neither fibrous septa nor lobulated internal architecture can be detected in any thymic carcinoma. If MRI is performed on a patient with anterior mediastinal tumors, thymic carcinoma may be precisely diagnosed when characteristic MR findings are demonstrated.