Anaplastic thyroid carcinomas (ATC) are undifferentiated tumors that show rapid progression and dissemination. The recent increase in knowledge about cancer stem cells has attracted marked interest in ATC, since these lesions are suggested to be closely related to thyroid stem cells (TSCs). Due to the rarity of ATCs, however, the gene expression patterns that characterize these lesions have not been fully clarified. Using real-time quantitative reverse transcription-polymerase chain reaction, we measured the mRNA expression levels of three representative genes, telomerase reverse transcriptase (hTERT), thyroglobulin and thyroid transcription factor 1 (TTF-1) in twelve frozen tissue samples of ATC and six cell lines derived from ATCs. All twelve ATC specimens and six ATC cell lines showed an increased expression of hTERT mRNA but the expression levels of hTERT mRNA did not show a clear difference between ATCs and other thyroid tumors. The mean expression level of thyroglobulin mRNA in the ATCs and ATC cell lines was <10(2) times higher than that in the differentiated thyroid carcinomas. All twelve ATCs showed a loss of TTF-1 mRNA expression, but two cell lines, TCO-1 and 8505C, expressed TTF-1 mRNA. In conclusion, ATC tissues and cell lines were characterized by the expression of hTERT and a loss of thyroglobulin expression, concordant with the general recognition. However, the loss of TTF-1 expression can not characterize ATC cells, as some ATC cell lines expressed TTF-1 mRNA abundantly. This information could contribute to clarifying the nature of ATCs and could be useful in detecting TSCs, which have not yet been identified.