Type II iodothyronine deiodinase (DII) messenger ribonucleic acid (mRNA) and its activity have been demonstrated in human normal brain. Although DII activity has been demonstrated in brain tumors, expression of DII mRNA has not been studied in these tumors. To investigate the mechanisms involved in the expression of DII activity in brain tumors, we studied DII mRNA and DII activity in astrocytoma (two cases), glioblastoma (three cases), and oligodendroglioma (one case). DII mRNA, the size of which was indistinguishable from that in control cerebral cortical tissue, was demonstrated in all of the brain tumors tested, although the intensity of the hybridization signal showed wide variation among the tumors. DII activity was also detected in all tumors. DII mRNA and DII activity were highest in the tissue from oligodendroglioma. A significantly positive correlation was observed between DII mRNA and DII activity in these tumors (r = 0.94; P < 0.01), suggesting that DII expression in brain tumors is regulated at the pretranslational level. The present results demonstrate, for the first time, that DII mRNA as well as DII activity are expressed in brain tumors, and that DII mRNA is significantly correlated with DII activity in those tissues.