Nine primary malignant lymphomas of the lung are described. Seven were localized lesions, and two were diffuse bilateral infiltrations. Histopathologic examination of these lesions suggests that they represent examples of the same entity and that it is a malignant lymphoma rather than a reactive infiltration or "pseudolymphoma." Histologic evidence of lymph node involvement is unusual, even in the presence of mediastinal or pleural infiltration. Immunohistochemical studies demonstrated a monotypic B-cell population in cases in which suitable tumor tissue was available for study (four of nine). Of the other five cases, two had a population of monotypic B lymphocytes in the peripheral blood, and in one of these and in one additional case gastric lymphomas subsequently developed. It is suggested that these lymphomas arise from bronchus-associated lymphoid tissue, which would explain both their histologic features and their tendency to remain localized to the lung for long periods.