Six patients with lymphomatous lesions primarily involving the pulmonary parenchyma were studied. In these patients, both the history and physical findings were vague and minimal. The laboratory findings also were nonspecific, although the findings of large multiple lesions in the lungs and pleural adhesion or effusion were more consistent with lymphoma. Histologic examination revealed lymphocytic infiltration of the pulmonary parenchyma in all six patients and presence of germinal or growth centers in some areas of the lesions in four. The hilar or mediastinal lymph nodes were not involved in five patients so examined. Immunocytochemical study of cytoplasmic immunoglobulin revealed monoclonal lymphocytic proliferation in five patients and negative staining in one patient. Clinical, histologic, or immunohistochemical studies alone may not be sufficient to detect all of the lymphomatous lesions. The combined use of all of these parameters is more advantageous for accurate diagnosis of these lesions. Treatment is surgical resection. Radiotherapy or chemotherapy are used when residual disease is present after surgery. Three patients died of disseminated lymphoma 96, 42, and 8 months after diagnosis, respectively, and three patients are still alive at 18, 24, and 4 months, respectively.