A series of thirteen cases of mediastinal masses resembling thymoma are described. These masses, which grossly, radiologically, and microscopically resemble thymic tumors, are found to be a form of lymph-node hyperplasia. The lesions are benign, with no evidence of recurrence or metastasis after removal. Clinically, most patients were asymptomatic, with the mediastinal shadow detected on routine chest X-rays. The lesions were located in various parts of the mediastinum, with most close to the tracheobronchial tree. Microscopically, the lesions showed hyperplasia of lymphoid follicles with germinal centers and marked capillary proliferation with endothelial hyperplasia. These features resemble the Hassall corpuscles of the thymus, but no keratin was found, and the lesions were not thymic in origin. The condition is not neoplastic and is characteristic of chronic nonspecific inflammatory processes, such as lymphadenitis. The lesions were not associated with Hodgkin's disease. The authors conclude that these lesions are benign hyperplastic lymph nodes, not thymic tumors. The findings suggest that the lesions are a form of chronic lymphadenitis, possibly due to a low-grade, nonspecific, chronic inflammatory process. The lesions are not typically found in other parts of the body, but similar findings have been observed in other lymph nodes. The authors also note that the lesions may be a reaction in a hemolymph node, but this is not confirmed.A series of thirteen cases of mediastinal masses resembling thymoma are described. These masses, which grossly, radiologically, and microscopically resemble thymic tumors, are found to be a form of lymph-node hyperplasia. The lesions are benign, with no evidence of recurrence or metastasis after removal. Clinically, most patients were asymptomatic, with the mediastinal shadow detected on routine chest X-rays. The lesions were located in various parts of the mediastinum, with most close to the tracheobronchial tree. Microscopically, the lesions showed hyperplasia of lymphoid follicles with germinal centers and marked capillary proliferation with endothelial hyperplasia. These features resemble the Hassall corpuscles of the thymus, but no keratin was found, and the lesions were not thymic in origin. The condition is not neoplastic and is characteristic of chronic nonspecific inflammatory processes, such as lymphadenitis. The lesions were not associated with Hodgkin's disease. The authors conclude that these lesions are benign hyperplastic lymph nodes, not thymic tumors. The findings suggest that the lesions are a form of chronic lymphadenitis, possibly due to a low-grade, nonspecific, chronic inflammatory process. The lesions are not typically found in other parts of the body, but similar findings have been observed in other lymph nodes. The authors also note that the lesions may be a reaction in a hemolymph node, but this is not confirmed.