The authors present a series of thirteen cases of mediastinal lymph node hyperplasia that closely resemble thymomas in gross, radiological, and microscopic appearances. These cases, reported from various institutions, were initially classified as thymomas but are now reclassified as benign lymph node hyperplasias. Clinically, most patients had no symptoms, and the masses were discovered incidentally on chest X-rays. The lesions were generally benign, with some existing unchanged for up to eight years without recurrence. Microscopically, the lesions featured hyperplasia of lymphoid follicles with germinal centers and marked capillary proliferation, leading to a resemblance to Hassall corpuscles in the thymus. However, the lack of keratin staining and the presence of plasma cells and eosinophils suggest a chronic inflammatory process rather than a neoplasm or thymic origin. The authors conclude that these cases represent a unique form of lymph node hyperplasia, likely resulting from a low-grade, nonspecific, chronic inflammatory process.The authors present a series of thirteen cases of mediastinal lymph node hyperplasia that closely resemble thymomas in gross, radiological, and microscopic appearances. These cases, reported from various institutions, were initially classified as thymomas but are now reclassified as benign lymph node hyperplasias. Clinically, most patients had no symptoms, and the masses were discovered incidentally on chest X-rays. The lesions were generally benign, with some existing unchanged for up to eight years without recurrence. Microscopically, the lesions featured hyperplasia of lymphoid follicles with germinal centers and marked capillary proliferation, leading to a resemblance to Hassall corpuscles in the thymus. However, the lack of keratin staining and the presence of plasma cells and eosinophils suggest a chronic inflammatory process rather than a neoplasm or thymic origin. The authors conclude that these cases represent a unique form of lymph node hyperplasia, likely resulting from a low-grade, nonspecific, chronic inflammatory process.