We report three cases of pulmonary involvement of non-Hodgkin's lymphoma in which immunophenotypic or gene rearrangement analysis of bronchoalveolar lavage (BAL) cells demonstrated monoclonality of T- or B-cell lineage. The first patient had T-cell lymphoma and developed pulmonary lesions. Surface marker analysis of the BAL cells revealed that CD8-positive lymphoid cells were dominant and Southern blot analysis of T-cell receptor gene detected gene rearrangement demonstrating monoclonality of T-cell lineage. The second patient presented with diffuse micronodular shadows on chest radiograph. Marked B-lymphocytosis in BAL fluid prompted us to analyze their clonality. The third was a case in which recurrence of primary pulmonary lymphoma was suspected. In the second and third case, the finding of marked increase in the number of CD 19-positive B lymphocytes with a single class of light chains proved a monoclonal population in BAL cells. With the review of other cases in our study and the relevant literature, we conclude that the clonal analysis of BAL cells is helpful in establishing the diagnosis of pulmonary involvement of T- or B-cell lymphoma.