We investigated the staining pattern of commonly used basal cell/myoepithelial markers, such as p63 (a p53-homologous nuclear protein), basal cell-specific cytokeratin antibody (34betaE12, K903), and smooth muscle myosin heavy chain (SMMHC) in benign and malignant bronchioloalveolar proliferations of the lung. We studied 85 lung lesions consisting of 35 bronchioloalveolar carcinoma, 30 well-differentiated adenocarcinoma, and 20 cases of benign lung lesions. In normal lung, p63, K903, and SMMHC decorated the basal cells of large and small airways and occasional cells of terminal bronchioles. In reactive processes, a distinctive staining pattern was present in 19/20 (95%) of the cases characterized by staining of basal cells of the airways and bronchiolar epithelium and squamous metaplastic epithelium for p63 and K903, whereas 12/20 (60%) stained with SMMHC. Respiratory ciliated cells, alveolar epithelial cells, and nonepithelial cells were negative. In bronchioloalveolar carcinoma, a discontinuous peripheral rim of p63-immunoreactive cells was retained surrounding and intermingled with the malignant bronchioloalveolar proliferation in 31/35 (88.5%) cases, SMMHC in 28/35 (80%) cases, and K903 in 20/35 (57%) cases. For adenocarcinoma, a majority of the cases (28/30, 93%) were negative for p63 and K903; however, SMMHC showed artifactual staining in the desmoplastic stroma in 6/30 (20%) cases. Our results highlighted the differential expression of basal cell markers across various bronchioloalveolar lesions. The staining pattern of basal cells in bronchioloalveolar carcinoma supports that these neoplasms may actually be carcinoma in-situ.