The expression of thyroid transcription factor-1 in normal and neoplastic tissues and cell lines of the human lung was investigated using immunohistochemistry and in situ hybridization in conjunction with reverse transcription polymerase chain reaction. In normal lung tissues, immunoproducts of thyroid transcription factor-1 were observed in the nuclei of alveolar cells and bronchiolar cells. Interestingly, in distal bronchioles, immunohistochemistry and in situ hybridization revealed that thyroid transcription factor-1 was present not only in nonciliated cells (Clara cells) but also in ciliated cells and basal cells. In neoplastic tissues, thyroid transcription factor-1 was demonstrated in adenocarcinomas and small cell lung carcinomas with high frequency: 96% and 89% of cases, respectively. Thyroid transcription factor-1 was not detected in squamous cell carcinomas and large cell carcinomas. The strong immunoreactivity of thyroid transcription factor-1 or simultaneous expressions of thyroid transcription factor-1 and surfactant protein A tended to correlate with the differentiation phenotypes in adenocarcinomas; they were more frequently present in the well-differentiated type than were moderately and/or poorly differentiated types. By reverse transcription polymerase chain reaction, expression of thyroid transcription factor-1 messenger RNA was observed in squamous cell carcinomas in addition to in adenocarcinomas and small cell lung carcinomas, and this finding was confirmed in the cell lines from squamous cell carcinomas. Only one case of 99 adenocarcinomas that originated in various organs other than lung and thyroid immunohistochemically expressed thyroid transcription factor-1. Our results suggest that thyroid transcription factor-1 can play an important role for the maintenance and/or differentiation process in bronchiolar and alveolar cells. Thyroid transcription factor-1 expression associates with histologic types and/or differentiation of lung cancers and can be a valuable marker for the better understanding of their biological nature and pathological behavior.