In diagnostic pathology thyroid transcription factor-1 (TTF-1) is used as a relatively specific and sensitive diagnostic marker of thyroid and lung adenocarcinomas and lung carcinoids but has also been demonstrated in minor proportions of carcinomas from other organs as well as nonepithelial neoplasms. Two antibody clones are widely used for TTF-1 demonstration, 8G7G3/1 and SPT24, the latter being the most sensitive. Few studies have addressed the occurrence of TTF-1 in central nervous system (CNS) tumors with highly divergent results, a major reason for which seems to be use of different clones. Based on multitissue blocks we analyzed the TTF-1 expression in a series of 155 CNS tumors comparing antibody clones 8G7G3/1 and SPT24 in optimized protocols on the Benchmark Ultra stainer. With clone SPT24 TTF-1 staining was observed in 13 cases (8%). Among astrodendroglial and oligodendroglial tumors, TTF-1 expression was found in 10 of 56 grades III to IV tumors (18%), as opposed to 0 of 47 grades I to II tumors (0%). The TTF-1 expression in positive tumors was generally weak to moderate and focal (mean histoscore 28, range: 2 to 120). TTF-1 positivity was inversely correlated to the expression of nestin. Among 52 other CNS tumors, TTF-1 expression was found in 1 of 3 central neurocytoma (the only CNS tumor with a moderate, diffuse staining), 1 of 18 ependymal tumors, and 1 of 5 choroid plexus tumors, whereas 4 pineal tumors, 11 meningiomas, 8 embryonal tumors, and 4 mixed neuronal-glial tumors all were negative. None of the 155 tumors stained with the 8G7G3/1 antibody clone. TTF-1 expression in primary brain tumors should be taken into consideration when interpreting brain tumors of uncertain origin.