A 60-year-old woman had an abnormal shadow in the right lower lung field on chest roentgenogram. Transbronchial lung biopsy revealed findings consistent with malignant lymphoma, and a right middle lobectomy was performed. Pathological findings showed that tumor cells had infiltrated the epithelium, forming so-called lymphoepithelial lesions. Flow cytometric analysis of the resected specimen revealed that B-cell associated antigens (CD 19, 20) were expressed, and that the tumor cells were CD 5-, CD 10-. A marked increase in the number of lymphocytes with an IgM kappa component suggested monoclonal origin for the tumor cells in the resected specimen. Southern blot analysis showed clonal rearrangement of the heavy chain of the immunoglobulin gene. A diagnosis of malignant lymphoma of bronchus-associated lymphoid tissue was made. This tumor was defined according to the revised European. American classification of lymphoid neoplasms as a marginal zone B-cell lymphoma.