Low-grade B-cell lymphoma of mucosa-associated lymphoid tissue (MALT) type represents one of the most frequent primary lymphomas in the lung. This study demonstrates the value of bronchoalveolar lavage (BAL) in the diagnosis of this entity. Three patients with biopsy proven MALT lymphoma in the lung were submitted to bronchoscopy. BAL fluid analysis showed an increased cellularity and a striking lymphocytosis. Cytological features consistent with a low-grade malignant lymphoma were present in two cases (medium sized lymphoid cells with an evident lymphoplasmocytoid differentiation, and irregular nuclear borders). In all cases, flow cytometry analysis showed a high percentage of cells expressing a B phenotype (52, 40 and 28%, respectively). In two cases, there was a striking monotypic expression of surface light-chain immunoglobulin (Kappa/Lambda ratio = 38 and 0.04, respectively). Bronchoalveolar lavage appears to be a valuable procedure in the diagnosis of low-grade B-cell lymphoma of MALT type in the lung.