Background: The aim of this study was to assess the correlations between the levels of blood glucose (BG) and the dose of radiation therapy (RT) to the central gustatory system (GS) in glioblastoma multiforme (GBM) patients. Methods: Thirty-seven GBM patients with regular blood glucose measurements were investigated retrospectively. 59.5% were female and 40.5% male with a median age of 64.3 years (range 27.4-85.6). Diabetes mellitus type 2 (DM2) history, BG levels and dexamethasone (DEXA) medication were assessed. The analyzed central gustatory structures were: solitary tract and nucleus, ventral posteromedial nucleus of the thalamus, sensory tongue area of the postcentral gyrus, anterior part of the insula, frontal operculum, amygdala, hypothalamus. These structures were delineated on magnetic resonance tomographies (MRIs) registered to planning-CTs. All GS doses were transformed in equivalent doses in 2 Gy fraction (EQD2). Results: Twenty one patients (56.8%) had at least one BG values over 200 mg/dl during RT. There was a difference between average BG in DM2: 192.8 mg/dl (±24.4) and non-DM2 patients: 145.7 mg/dl (±39.5; p = 0.01) but no significant difference in daily DEXA medication - DM2 patients: 7.9 mg/d (±1.9) vs. non-DM2: 9.3 mg/dl (±5.7; p = 0.29). The EQD2 Dmean to the total GS was 36.0Gy (±8.6 Gy). There was a tendency for a higher increase in maximum BG values with more radiation dose to the total GS (b = 1.9, R2 = 0.103, p = 0.06). Conclusion: BG levels in GBM patients are in direct correlation to the dose of RT applied to the central GS. GBM patients that undergo RT should thus be closely monitored for changes in BG levels during and after the radiation.