A successfully treated case of cyclophosphamide pneumonitis, which developed during treatment for a leukemic phase of B-cell non-Hodgkin's lymphoma, is documented. Subtyping of lymphocytes in the peripheral blood and the bronchoalveolar lavage fluid easily excluded a possible diagnosis of pulmonary involvement of lymphoma. Bronchoalveolar lavage is a useful approach to determine the correct etiology of pneumonitis.