A patient with bilateral interstitial infiltrates was evaluated by bronchoalveolar lavage prior to open lung biopsy. The patient was found to have lymphocytic alveolitis (34 percent lymphocytes) in which 43 percent of lymphocytes were B cells. A clonal proliferation of these B cells was suggested by the finding of monoclonal kappa light chain on the B lymphocytes. A suspected diagnosis of primary pulmonary non-Hodgkin's lymphoma was later confirmed by open lung biopsy.