The miR-17∼92 cluster is thought to be an oncogene, yet its expression is low in glioblastoma multiforme (GBM) cell lines. This could allow unfettered expression of miR-17∼92 target genes such as connective tissue growth factor (CTGF; or CCN2), which is known to contribute to GBM pathogenesis. Indeed, microRNA-18a (but not other miR-17∼92 members) has a functional site in the CTGF 3' UTR, and its forced reexpression sharply reduces CTGF protein and mRNA levels. Interestingly, it also reduces the levels of CTGF primary transcript. The unexpected effects of miR-18a on CTGF transcription are mediated in part by direct targeting of Smad3 and ensuing weakening of TGFβ signaling. Having defined the TGFβ signature in GBM cells, we demonstrate a significant anti-correlation between miR-18 and TGFβ signaling in primary GBM samples from The Cancer Genome Atlas. Most importantly, high levels of miR-18 combined with low levels of the TGFβ metagene correlate with prolonged patient survival. Thus, low expression of the miR-17∼92 cluster, and specifically miR-18a, could significantly contribute to GBM pathogenesis.