A total of 211 cases of primary lung adenocarcinoma were tested for expression of gross cystic disease fluid protein-15 (GCDFP-15) and only 11 cases (5.2%) were positive. The cases occurred with an equal sex distribution in older individuals whose carcinomas were frequently identified on screening radiographs. The adenocarcinomas were peripheral lesions and had an average size of 2.9 cm (range, 1.1 to 7.0). Histologically, they were usually mixed acinar and papillary adenocarcinomas with abundant extracellular mucin production, with the neoplastic cells having a polygonal shape, round to oval nuclei, diffuse powdery chromatin, and abundant eosinophilic granular cytoplasm. Clear cells and apocrinelike cells with prominent central nucleoli were common. GCDFP-15 was expressed in conjunction with thyroid transcription factor-1 in 81% of cases and synaptophysin was seen in 65%. Estrogen and progesterone receptors were not expressed. EGFR gene amplification and mutations of exons 19 and 21 were rare. KRAS mutations and HER2 gene amplification were not seen. This report details the first 11 cases of pulmonary adenocarcinoma to express GCDFP-15 and their distinctive morphology with frequent mucin production and coexpression of thyroid transcription factor-1 and synaptophysin.