Models describing progression in the genetic derangement of glial tumors have shown chromosomal loss and gain occurring most frequently in high-grade lesions, suggesting that identification of these aberrations may be prognostically significant. In this study, Fluorescence in situ hybridization (FISH) has been used to determine, and to confirm, loss and gain of chromosomes 1, 8, 10, 12 and 17, in formalin-fixed, paraffin-embedded brain biopsy tissue taken from 60 brain gliomas submitted to surgical resection or stereotactic biopsy. FISH analysis may be a valuable adjunct to histological grading. The results showed that this molecular cytogenetic technique is an important clinical and experimental tool that provides new insight on genetic alterations, confirming gain and loss of genetic material that occurs at the initiation and progression of human glioma. Our data suggests that potentially useful prognostic information may be obtained through this approach. Monosomy 10 was the most statistically significant negative predictor of patient survival, showing a significant correlation with the histological grading.