Peripheral pulmonary adenocarcinomas (PPAs) often demonstrate a bronchioloalveolar component, with or without glandular differentiation. PPAs can be nondescript, mucinous, or show features of Type II pneumocytes. Particularly, mucinous lung carcinomas can resemble gastrointestinal metastases. Previous reports suggested that patterns of keratins 7 (K7) and 20 (K20) differ in pulmonary tumors versus enteric metastases. These studies, however, often failed to specify the precise morphotypes of PPA. Thus, we undertook this evaluation of PPAs with different histologic images. Thirty-nine cases were retrieved from institutional files; all were confirmed as primary tumors by clinicopathologic and radiographic review. Cases were classified as Type I (mucinous) bronchioloalveolar carcinoma (BAC1); Type II (nonmucinous) bronchioloalveolar carcinoma (BAC2); conventional PPA with BAC1-like areas (PPA1); or conventional PPA with BAC2-like foci (PPA2). Immunostains were performed for K7, K20, carcinoembryonic antigen, CA19-9, tumor-associated glycoprotein-72, surfactant apoprotein-A, and the c-erbB-2 peptide. BAC1 and PPA1 failed to express surfactant apoprotein-A, and BAC2 also consistently lacked K20, whereas 28% of PPA2 lesions were labeled for K20. All of the other determinants, however, were seen in variable proportions in each subgroup of PPA. Primary BAC1 and PPA1 resembled enteric adenocarcinomas immunophenotypically; on the other hand, BAC2 demonstrated a pattern of protein expression similar to that of Type II pneumocytes. PPA2s are a diverse group of neoplasms, and a subset of PPA2 does show K20 reactivity, as would be expected in metastatic enteric carcinomas. Thus, immunohistochemical data on PPAs must be interpreted carefully and only in clinicopathologic context. With respect specifically to primary pulmonary mucinous tumors, there still seems to be no uniformly reliably marker that will always allow the exclusion of metastatic enteric tumors.