We studied the staining patterns of bronchioloalveolar carcinoma (BAC) with antibodies to cytokeratin (CK) 7, CK20, and thyroid transcription factor-1 (TTF-1) to determine the diagnostic usefulness of this panel in differentiating BAC from metastatic adenocarcinoma in material obtained by fine-needle aspiration biopsy (FNAB) of the lung. We identified 16 cases of BAC. Of these, 6 were mucinous, 4 were nonmucinous, and 6 were mixed with focal mucinous differentiation. Immunohistochemical analysis with antibodies to CK7, CK20, and TTF-1 was performed on cell-block sections. Of the 6 mucinous BACs, 4 (67%) were CK7+, CK20+, and TTF-1-. All 4 nonmucinous BACs were CK7+ and CK20-, and 2 (50%) were TTF-1+. All 6 mixed BACs were diffusely positive for CK7 and focally positive for CK20; 5 (83%) were TTF-1+. Nonmucinous BACs display CK7, CK20, and TTF-1 immunoreactivity similar to conventional pulmonary adenocarcinoma. Mucinous and mixed BACs have an immunohistochemical phenotype that is different from that of conventional pulmonary adenocarcinoma. Knowledge of these staining patterns is crucial for distinguishing mucinous and mixed BACs from metastatic adenocarcinoma involving the lungs.