This review examines the apparently paradoxical conversion of transforming growth factor beta's (TGFbeta) regulatory role as a growth inhibitor among normal glial cells to that of a progression factor among glioblastomas (GM). In vitro, TGFbeta functions as an autocrine growth inhibitor of near-diploid gliomas of any grade. In contrast, hyperdiploid glioblastoma multiforme (HD-GM) cultures proliferate in response to TGFbeta, which is mediated by induction of platelet-derived growth factor B chain (PDGF-BB). The dominant hypothesis of TGFbeta's pathogenetic association with malignant transformation has been predicated upon acquisition of resistance to its growth inhibitory effects. However, the lack of obvious correlation with TGFbeta receptor (TbetaR) expression (or loss) between the HD-GM and the TGFbeta-inhibited GM cultures suggests the existence of intrinsically opposed regulatory mechanisms influenced by TGFbeta. The mechanism of conversion might be explained either by the loss of a putative tumor suppressor gene (TSG) which mediates TGFbeta's inhibition of growth or by enhancement of an active oncogenic pathway among the HD-GM. The frequency of mutations within glioma-associated TSG, such as TP53 and RB, suggests that defects in TGFbeta's inhibitory signaling pathway may have analogous effects in the progression to HD-GM, and TGFbeta's conversion to a mitogen. Alternative sites of inactivation which might explain the loss of TGFbeta's inhibitory effect include inactivating mutation/loss of the TbetaR type II, alterations in post-receptor signal transmission or the cyclin/cyclin dependent kinase system which regulates the phosphorylation of pRB. Loss or inactivation of a glial TSG with a consequent failure of inhibition appears to allow TGFbeta's other constitutive effects, such as induction of c-sis, to become functionally dominant. Mechanistically, TGFbeta's conversion from autocrine inhibitor to mitogen promotes 'clonal dominance' by conferring a Darwinian advantage to the hyperdiploid subpopulations through qualitative and quantitative differences in its modulation of PDGF-A and c-sis, with concomitant paracrine inhibition of competing, near-diploid elements.