Alterations in the signal transduction pathways are key mechanisms in the pathogenesis of de novo glioblastoma multiforme (GBM), which are also involved in the resistance to chemo- and radiotherapy. Here, we analyzed the methylation status and mRNA expression of suppressor of cytokine signaling (SOCS)1-2-3, 3 of the most important inhibitory molecules of the signal transduction circuitry, in 46 GBM specimens. The relationship between methylation status of SOCS1-2-3 and clinical outcome was investigated. Using methylation-specific PCR (MS-PCR) and sequencing, after bisulphite modification, we found that the promoter of SOCS1-2-3 was methylated in 24, 6.5 and 35% of GBM, respectively. Real-time analysis showed that in methylated GBM, mRNA expression for SOCS1-2-3 was reduced by 5, 3 and 7-folds, respectively, when compared with unmethylated GBM. Moreover, methylation of SOCS3 promoter significantly associated with an unfavorable clinical outcome (p < 0.0002). Our data suggest that methylation of SOCS3 may be involved in the pathogenesis of GBM and in the resistance of this neoplasm to conventional treatment.