Thyroid transcription factor-1 (TTF-1) is a sensitive marker for pulmonary and thyroid adenocarcinomas. The aim of this work was to determine its usefulness in distinction between primary and metastatic lung adenocarcinomas. We have examined the expression of TTF-1 in 100 solitary pulmonary nodules. They included 50 stage I peripheral primary bronchial adenocarcinomas (30 men, 20 women, mean age: 60 years) and 50 metastatic pulmonary adenocarcinomas (21 men, 29 women, mean age: 57 years) of different origins, such as breast (13), colon (13), rectum (13), kidney (7), stomach (2), and thyroid gland (2). TTF-1 immunohistochemistry was performed on formalin-fixed, paraffin-embedded tissues. In primary bronchial adenocarcinomas we found immunopositivity in 46/50 cases, among them 30 cases showed strong nuclear immunostaining. In four primary adenocarcinoma cases the observed immunopositivity was localized to the cytoplasm. Out of the metastatic adenocarcinomas all but the 2 thyroid cancers were negative. Both thyroid tumors showed strong immunopositivity. Our results confirm that TTF-1 immunohistochemistry is a very sensitive and highly specific method in the differential diagnosis of primary and metastatic lung adenocarcinomas and should be used in the everyday clinical practice.