Among 69 cases of thymoma, 16 of the lesions were not detected immediately in the first Mayo Clinic posteroanterior radiographs. Often they resembled right heart enlargement, their margins overlay the shadow of the right heart or hilus, or they were too small to be seen. A helpful sign was lobulation of the mass. Used selectively within the series, lateral views, computed tomography (CT), oblique views guided by fluoroscopy, and plain-film tomography contributed variously to accuracy of diagnosis and description. Until a determination is made, application in the order listed is recommended. Lateral tomograms often showed details of small, isolated abnormalities in the anterior mediastinum. Although CT did not reveal any lesions that were not suspected from other radiographic studies, it did show important anatomic relations. Early detection is important: 29% of the lesions were malignant, including 22% of those smaller than 5 cm. However, diagnosis was delayed after initial radiologic investigation (either elsewhere or at Mayo Clinic) in 17 cases (25% of the series), for intervals averaging 41 months. None of the imaging techniques was accurate in predicting malignancy, although CT best demonstrated adherence and invasion.