Background: Genome-wide transcriptional studies (TCGA and others) have identified distinct GBM molecular subtypes, but to date this has not translated into prognostic or therapeutic implications. Bevacizumab has emerged as a new treatment option for GBMs, although a survival benefit has yet to be demonstrated in unselected patients (pts). We analyzed outcomes from a prospective phase II trial in newly diagnosed GBM treated with hypofractionated stereotactic radiotherapy (HFSRT) combined with temozolomide and bevacizumab, and correlated with GBM transcriptional subclasses. Methods: Pts with newly diagnosed GBM with tumor volume < 60cc were eligible. Treatment consisted of HFSRT (6x6 Gy to contrast-enhancing tumor and 6x4 Gy to FLAIR hypersignal with dose painting), concomitant with bevacizumab (10 mg/kg Q2 weeks) and temozolomide (75mg/m² daily), followed by standard adjuvant bevacizumab/ temozolomide. Primary endpoint was 1-y overall survival (OS). To establish TCGA transcriptional subclasses, mRNA from formalin-fixed paraffin-embedded tissue blocks was analyzed with a validated, 151-probe Nanostring gene expression assay. Results: A total of 40 evaluable pts were accrued, achieving a 1-y OS of 90% (95% CI 76-96), 2-y OS of 32% (95%CI 19-47) and median OS of 17.4m. The molecular subclass could be defined in 31 pts, as follows: Mesenchymal: 14 (45%) pts, pro-neural: 8 (26%), classical: 7 (23%), neural: 2 (6%). The pro-neural phenotype was associated with reduced overall survival: median OS of 13.5m vs 21.2m for non pro-neural tumors (univariate: p = 0.015; multivariate: p = 0.003). MGMT promoter methylation did not predict survival (p = 0.13). Conclusions: We provide proof-of-principle that GBM transcriptional classification is biologically and therapeutically relevant, identifying non pro-neural GBMs as the best candidates for bevacizumab treatment. Our findings imply that angiogenesis and tumor invasion mechanisms in proneural tumors may be distinct from other subtypes, and we suggest such pts should not be offered bevacizumab treatment upfront. Future randomized trials focusing on non-proneural tumors may finally demonstrate a survival advantage for bevacizumab in GBM. Clinical trial information: NCT01392209.