An 80-year-old man was admitted to our hospital with complaints of left-sided chest pain and swelling. A chest roentgenogram revealed a left chest-wall mass. Examination of a biopsy specimen of the mass led to the diagnosis of Non-Hodgkin's lymphoma of diffuse mixed-cell type. Immunohistological examination revealed that it was B-cell type. The chest-wall mass was markedly smaller after radiation therapy, and the patient had a complete remission. About 1 year and 10 months later, Non-Hodgkin's lymphoma developed in the right femur. Combination chemotherapy (cyclophosphamide, adriamycin, vincristine, and prednisolone) and radiation therapy were given. The patient went into a complete remission, and is alive at the time of this writing, 2 years and 6 months after he was first treated. Almost all malignant lymphomas of the chest wall arise in cases of chronic tuberculous pyothorax or tuberculous pleuritis, but in this case there was no preceding disease. When the chest roentgenogram shows a chest-wall mass, we must consider the possibility of malignant lymphoma, even if there is no preceding disease.